Chain restaurants are a fact of life in Europe as well as the rest of the world. Weinerwald was all over Germany years before McDonalds moved in, but seems to be fading now. McD's, Burger King, KFC, Subway, etc have ll been in Germany (and much of Europe) for over 30 years, brought in part by the presence of large local populations of American military personnel. The area around Kaiserslautern, for example, once held over 300,000 Americans, and there were a number of other locations that were almost as heavily populated. When you consider that cities in the USA with a population approaching that usually have a considerable number of fast food choices it's no surprise that they could find a foothold both on and off base in other countries.
I don't really consider those to be Denny's equivalents though, and I think the reason "sit down" chains struggle is more due to cultural differences than anything. Europeans do not, as a rule, eat anything like an American breakfast. And since for many countries the main sit down meal is lunch most places have a number of local places that fill that niche. After work places, like TGIF tried to be, are the pubs, bierhalls, and other traditional gathering places, which most chains never really transitioned to.
There are some Internationally known chains that are moving into traditional neighborhoods; The Hard Rock Cafe is one of those. But I'm not sure how long that will last, as they seem to be rather dated even in the US. And TGIF and other places like that are being started overseas, in some cases in the same manner as the initial franchises. There is a TGIF (for example) in Qatar. One thing I seem helping such businesses is the spread of international hotels. Chains like Marriott (or Bonvoy now that it's absorbed a dozen other international chains) import culture as well as living space, and I fully expect to see Chinese versions of both the hotels and eateries now days when I travel.
Successful franchises have to adjust to local mores. McDonalds serves beer in Germany, Burger King offers mayo with the frys, even Starbucks changes the menu or folds (ask them about Italy sometime). And there are locally grown franchises in other countries. People in this thread have mentioned a number. Some may even make it to the USA.