How to switch languages quickly and easily?

Basically no problem. What you are describing is what I call "linguistic interference." I run into that problem too , it's rare, still it does take place between French and German.

My solution (far easier said than done) is keep practicing in the language , pound away at it, which includes practicing writing it properly (the key word here) until you have internalized it, flipping between it and English almost instantly, thinking in the language.

but I only seem to have one slot in my head for one foreign spoken language at a time

I speak six languages ​​myself, and if you were to put six people next to each other, each speaking one of those languages, I would answer them one by one in their own language.
The only difference might be the speed of the conversation which might be a bit slower in the languages ​​I use the least or that some answers might include less complicated words because the amount of vocabulary known varies somewhat between languages. And of course, some conversations may contain more or fewer grammatical errors.

The blocking effect that is overpowering you is probably psychological. The fear of failure, being misunderstood, and making grammatical errors are the three most common reasons why people are afraid to switch to another language. Making mistakes feels like failure. And that seems like the "end of the world" to us.
Then there's the inevitable fear. Fear reduces your brain's ability to function properly. This means that if you're afraid to speak another language, your brain can't process the language properly.
The fear of being judged is usually present as well. Most of us are afraid of being judged. The same applies to foreign languages, because we often think others will judge us for our grammatical or pronunciation errors.

I know what you mean.

I speak two languages, and in school or in life studied five more (none of those five can I speak with any fluency, though I have phrases in all of them or random verb conjugations, vocabulary in a few domains, or whatever).

But there seem to be three buckets in my brain.

One that is labeled "native language" (English) and that I can speak without thinking about.

A second one that is labeled "French" - I can prerry much pull out of this one what I want. Not 100%, but enough to live and work and do my errands.

Then there is that third one, labeled "foreign language." This one is the problem. If I am in one of these countries, my brain knows it will need to reach in here for what I need.

But !!! Everything in there is all jumbled up. German is as foreign to me as Italian which is as foreign to me as Hungarian which is as foreign to me as Russian which is as foreign to me as Spanish. So when I tell my brain "it's time to use a bit of this foreign language now," I am as likely to come up with old Spanish phrases from high school as something from the Italian news i watched last week. I will think "puedo" is a darned good substitute for "posso" and not understand why my mother-in-law is looking at me perplexed.

That bucket just has way too many incremented pieces in it. I reach in, hoping to get something appropriate, but it's absolutely hit or miss.

One that is labeled "foreign language" and to which I dip in (undoetunatelh

Quickness comes from use and exercise. In the Alps there are several areas that use two or more languages in everyday life - think Tyrol (the Italian section is German/Italian bilingual, actually trilingual as there are Ladino areas; but also the Austrian Tyrol as a lot of Italian speakers) or Switzerland (four official languages, and Romansch has several dialects almost mutually non understandable).

In these cases I have often heard periods beginning in a language and ending in a different one; or single words may being lent from one language to another (for example, Swiss Italian speakers say "Azione", from the German "Aktion" for a premium deal in a shop, while the standard Italian is "offerta speciale"). The extreme case was when I stood listening for several minutes a supermarket cashier in Disentis, Switzerland, interacting with her clients, without being able to realize if she was speaking Romansch or German (two widely different languages) as everything was throughly mixed.

Use and exercise in language comes from necessity. If you have to deal with a problem you will have a powerful reason to speak in the language of the problem. In time, you become multilingual.

I absolutely know what you mean. I seem to only have room in my brain for one non-native language at a time. I think it gets harder the older I am. I think I really focus on ____ language in the time leading up to and during the trip so I kinda purge my foreign language memory banks of everything else temporarily.

And traveling from Italy to Spain - one would think it was easy, but I end up speaking the wrong language half of the time.

It can help to imagine separate boxes in your brain for each language. I speak 4 languages and don't get them mixed up, but if I have to interpret from French into German (neither of which is my first language, rather my third and fourth languages) I do find it cumbersome. But practice is the answer to most of these.

Lavandula

Ha ha! This has happened to me as well, and I've learned to just accept it. I'm far from fluent in anything but English, but we always learn some language for the country we're visiting next. A few years ago I learned some Italian and some French, but pulled the wrong one out in Florence, my first stop. When I thanked a woman in French, she immediately responded in French. Voila! I do find that starting the day by greeting my husband in the appropriate language helps cement my location.

Ha, ha! This has happened to me too.
Spanish is my native language and when I was first learning English, it would take me a while to translate every word in my brain before speaking it.
After many years of practicing, I don’t think about it and can switch easily from Spanish to English, and vice-versa.

I learned French many years ago (high school) and I also understand and speak a bit of Italian, but my brain definitely needs more practice in these languages.

Back in 2018 while visiting Sicily, I was reading a sign in Italian and a family approached me and asked me a question in French.
I understood the question but the words that came out of my mouth were in Italian!

@ Windyram...I marvel at your linguistic ability in knowing those 6 languages.. Your point is well taken especially that pertaining to the factor of fear. My question to an interlocutor on the topic of foreign languages is, "what are you afraid of?

In Germany I have been in situations apart from that connected with the usual activities of a traveler , such a in pharmacy, doctor's office in need of a his prescription , and also in a police office in a small town 3-4 KM from the German-Dutch border in the Lower Rhine area. I assumed that my interlocutors all knew English well (especially the pharmacist and the doc) or enough to engage in basic communication with me.

Upon seeing the US passport none of them switched over to English, just as I had expected, plus I had no desire to speak it anyway, thinking it's not going to do me any good by speaking English as a linguistic cop out. Language-wise I learned something from these encounters, which all went quite smoothly with no problems at all.

@Fred ... it is not that exceptional. Belgians automatically learn a second language very early in school (dutch for the Walloons, french for the Flemish). And watching TV, listening at music and playing videogames, the children get familiarized with english even faster (with a slight delay for the Walloons because many films are dubbed into French in their region, and they are more oriented towards France for music and video games).
At 16y old, a third language is added in school. While most teenagers will choose English, I took German, living myself close to the border and out of respect for the small german community in Belgium (war exchange of territory). So it was my 4th language, while I admit that it is the reason for my somewhat weaker performance in English (but not afraid to make mistakes).
And just out of love for languages, and travelling and taking part in conferences abroad I did a special effort for Italian and Spanish... and with me many belgians will run an equal path (not to forget that the best students even familiarized themselves with latin and greek).
... It's not just fries, beer and chocolate that characterize us 😉.

I know the feeling, but I have a different problem. I learnt French at school for 2 years then added a year a couple of decades later when I was hoping to go to Europe. Several more decades and I now have a beginners level of French, plus the same in Thai. My problem is, my ever helpful brain only recognises Australian or Foreign - so when I recently ran across a French group in Thailand and tried to have a brief chat in French, whenever I couldn't recall the French word, my brain immediately offered the Thai word, which my mouth gratefully accepted, leaving us all confused. The French group quickly switched to English and we conversed in our common second language.

@ Windyram... From your geographic (and historical too) clues, you must live in the region of Eupen.

On my 2023 trip I talked this young guy from Belgium in June during a ride on a regional train , all locals packed in like sardines, to Nürnberg. This "kid" early 20s was sitting on the floor , so packed was the train, got in a conversation with a German woman old enough to be his mom, who along with numerous others was standing. I luckily got a seat in their area, but was prepared to stand for ca, one hour or so.

Everyone could hear their conversation, all in German, obviously. I thought the Belgian kid was German (wrong) based on his accent and fluency. The woman might not have thought so, she asked him where he was from, (wo bist du her?) , he told her the name of the town in Belgium. At that point I got in on this and asked him in German if his town was located in Eupen. I told him I was from California, he did not change over to English, showed on is phone the location of his village. Never heard of it , either historically or geographically.

A very pleasant and enlightening encounter.

From your geographic (and historical too) clues, you must live in the region of Eupen.

Oh no, not in Eupen, because then German would have been my mother tongue or my second language, not my third.
I'm a real flemish guy, more to the north, but in the eastern part of Flanders, closer towards the german border.

Interesting country hé :-) https://en.wikipedia.org/wiki/Communities,_regions,_and_language_areas_of_Belgium

One country, three regions and three communities, all with their own governments and councils. Sometimes, it take months (or should I say years) before international journalists understand our system. You should see their faces when they get their first briefing when appointed to Belgium.
Currently, the new US ambassador Bill White is still struggling with it ...

Honestly, it can be very hard for most people unless you’ve spoken both languages for a good number of years.

I’ve learned Spanish more than German but I fing myself tending to speak DeutSpan. Interspersing words from both languages. It does take practice for most people but there are some rare people like the young man worked at the Belgium train information booth who could speak three languages without missing a beat. So jealous.

I am much like Kim with four different languages residing in my brain. If I am reaching for one of my 3 non native languages I’m never quite sure if enough words will pop up or be strung together with the matching conjunctions to make sense. It helps to practice more before going to a region by speaking and reading in that language. I begin to formulate some key phrases in my head. I had HS French, still most difficult to listen and respond to, I had intensive German classes while we lived in Germany for 6 months a long time ago and conversational German in other years. When we moved to California 40 years ago I took Spanish classes to be TOFL certified. This year we will be in Germany for 5 weeks with a bit of time France. I’d better start a refresher routine.

I’ve got heather’s problem. I know 3 languages--not fluent by any means, but I do ok. I learned French first, so that’s my brain’s go-to foreign language whether or not the person is speaking French to me! Now, before we go to French, Portuguese or Italian-speaking countries, I talk to myself in that language, watch movies, review my books, and try to converse with a native speaker if one lives nearby in hopes of jamming it into my brain and ear, and to set the others aside. But before, and sometimes in spite of doing that, I sometimes still manage to mix all three up so badly that one sentence containing a bewildering combination of languages. My self-exasperation is generally voiced in English. I apologize, explain to my interlocutor that I am learning all three, stop, regroup and continue on. I got over the fear of making mistakes a long time ago. Native speakers generally aren't bothered by it. On the contrary, they are patient and happy I am making the effort.

@ Windyram..... Thanks for your denoting your geographic location. .

Re: "three regions and three communities " and three official administrative languages too in one country. You have double that number in languages ranging I presume from fairly fluent to native fluency.

On the singular topic of language acquisition , I can only claim two ranging from fairly fluent (German) to fluent English, my native language pertaining all three aspects, writing, speaking and reading.

I too seem to have one slot in my brain for the "foreign language" chip. I speak very confident German and Italian, along with a smattering of other things. This summer, our German twin town sent a delegation to my Italian village. I was drafted to help communicate. Without even noticing, I would speak Italian to the Germans and German to the Italians and then try to translate for my English monoglot friends. Never a glitch with English but the German and Italian were fighting for that one slot! It was hilarious but annoying. More practice required!

Nelly,
probably hilarious but annoying in your mind ... not for the people around you, who will appreciate and truly value your knowledge of these three languages.

Within the animal kingdom, each animal speaks only one language. We humans also belong to the animal kingdom... much praise to those who distinguish themselves by speaking more than one language!

My advice is practice, practice, practice. The only two people I know who could switch languages at will were my grandmother and her sister. They would switch between 4 languages while talking on the phone to each other. They were convinced their conversations were being monitored. Like others I only have two slots for languages. English (which really isn't my first language) and other. If spoken to in French, for example, I may respond in either German or Spanish. Whatever comes out first.

You just have to have enough exposure to complete phrases or sentences to get them ingrained in your head, even temporarily. if you have to keep going back to English and finding the right word, it really slows you down. I remember having a conversation in Alsace in mixture of German and French and it worked well, since they were all bi-lingual. Whatever works.

"...practice, practice, practice." How utterly true in terms of speaking, writing, and reading.

You keep practicing , pound away at it until you have internalized it, dream in the language, and, above all, able to switch back and forth between 2 or even 3 languages with ease without running into linguistic interference .

I find that translating in your head is part of the problem. If you find yourself thinking. "baum means tree", or "Gelb means yellow", or any variation, that's a problem. Baum is baum, gelb is gelb. Quit translating.