I don’t really know how languages work in the feddiverse but I’ve heard that not setting the post langauge at all can result in your post being hidden from most users. So I always set my posts’ language to English. I find it a little silly that I have to scroll through a giant list of all possible languages to find mine. I have configured the language on my account so t should default to that. Or at least it should be at the top of the list.
In the settings there was a way for me to remove all the languages from the list except for undetermined.
I think everyone can read undetermined, but if you choose your language people can then use that to filter. But again I’m not really sure. I stopped selecting my language as I can’t change the default and I have no problems with visibility of my posts.