Spain and France

Train would be good but you can fly to Paris from Barca.

Most of the route between Barcelona and Paris is in France, but I suppose it would be possible to loop in a visit to San Sebastian. (I've never been, but it's supposed to be one of the world's great food cities, and a beautiful coastal spot with the Basque culture being markedly different from other regions of Spain.)

You can take a high-speed train from Barcelona to Paris in about 7 hours, so realistically, you’re losing most of a full day just travelling there and back. Is it really worth trying to squeeze Paris into this trip? Honestly, both Barcelona and Paris deserve at least 4 or 5 days each if you actually want to experience something meaningful instead of just collecting train stations.

I’m just wondering what you mean by “nice places along the way.” Since you mentioned you only have 7 days total, how much time are you planning to spend in Barcelona and how much in Paris? If Paris is part of the plan, adding extra stops in between might leave you with very little time in each place.

Just one note on the language side: Barcelona is the capital of Catalonia, where Catalan is the local language. Yet Spanish is also co-official and spoken, especially in the Barcelona metropolitan area, where there has historically been significant immigration from other Spanish-speaking territories, so your mother should have no problem communicating.

That said, the rest of the train journey toward Paris passes only through Catalonia until you reach France, and the everyday language used across most of that area is predominantly Catalan. Even in the south of present-day France, there are some 'departments' (think "counties") where Catalan still has a strong cultural presence alongside French, as that region historically formed part of Catalonia. This isn’t to say you can’t communicate in Spanish, but the most normal thing is that you’ll find people initially, and most of the time, speaking Catalan.