I would say that for long distance trains only English and the local language should be enough.

For international trains, we should have all languages of all traversed countries and English. So for example a train from Paris to Frankfurt should have announcements in French, German and English (and it is actually the case for that train, I already rode it).

But for example, the Berlin - Warsaw train has only English announcements besides the local language depending on the country the train is in (so no Polish when it is in Germany, and no German when it is in Poland), I consider this to be wrong. It should have announcements in Polish, German and English for the whole route.

Agree with your last point. That's a weird choice. At least the stops either side of the border are guaranteed to have people who natively speak the other language.

I seem to recall lines in Belgium that do announcements is 4 languages: french, Flemish, German, and English.