It is a bit rude to people who don’t understand the language, but at the same time, sometimes it is just easier to communicate in your native language (or one you’re otherwise more familiar with than English). I understand that people with a strong language background (such as German or French) prefer their own language.
Its OK to link back and forth. Say you read something in English, you can link to it in a German discussion. Vice versa can also happen. But we need to respect that most people here, speak their own native language. The main language on forum is English, and so threads are in English by default, with non-English elsewhere. With that being said, again, I understand that people with a strong language background (such as German or French) prefer their own language. So it can sometimes happen.
From the PoV of the German speaker who also speak English however (a prominent group here):
What I have lately seen a few times however was that posts were moved into topics that are written in a different language - mostly German posts into English topics.
This example makes sense for them, as its just noise to follow two threads about the same subject.
In my eyes, we can do two things:
We accept the German-English is the common culture here, and therefore we’re OK with such merges.
We strictly separate such, and have English as the default language.
That seems to be, in essence, what you’re asking?
I have no strong opinion that question. Keep in mind, we are in Europe. There are a lot of people in Europe, who speak a lot of languages (and my native language ain’t one of 'em). Dominant languages in Europe are (in no particular order) German, French, and English. On this forum, I’d say it is English and German (not necessarily in this order). If Fairphone expands to Scandinavia, I would expect the people there prefer English over German.
I’d say, we as a community need to follow the above, loosely, and not become angry but compassionate. Not everyone is able to express themselves well, let alone in a specific (non-native) language. Don’t assume malice, assume at worst mere incompetence.