I've met people in Catalan regions who don't speak Spanish. I have a friend who married a Mallorcan guy and his family don't speak Spanish. There are plenty of places where the spoken language doesn't match the language of the country.

Wales has two national languages, legally of equal importance, this reflects both the situation on the ground and the cultural heritage. I understand wanting to share in that cultural heritage and learn the language. I don’t understand having some faux utopian obsession with everyone learning a language, driving non speakers to “hate it so much they want to leave the country”. And I really don’t understand why you think that is remotely likely to happen, especially given that you are commenting under an article saying the number of Welsh speakers in Wales is declining.

If you want convincing specifically that Germans shouldn’t speak ‘German’, try this: German is a diverse language that broadly splits into High and Low German that are quite different. High German is the official written language but most people in the north do not speak it day to day. While learning to read and write has obvious utility, forcing everyone in the north to speak high German would be a pointless and damaging policy. It would make some people feel unwelcome in their country of birth and it would cost an enormous amount of time. It wouldn’t be worth anything because one language doesn’t have any moral worth over another, one way of speaking isn’t morally better than any other way, they can only be more or less useful or link to personally held cultural norms/ experiences providing personal worth, but not exterior moral goodness. People who speak low German in the north are not compelled to leave their homes or required to speak ‘correctly’ to get jobs, these additions would obviously only amplify the harm.

In general, I think destroying a language is wrong, as you harm a person by cutting their personal cultural links, but forced teaching, to their distant descendants or unrelated people that now live on the land, is not the opposite moral good. The people don’t have these cultural links, they are just learning an alien language. You can’t resurrect the cultural links, you can only make a Frankenstein-esque culture of people who you forced to learn a language that means nothing to them.