I’m assuming that this means that if you select a language, you will only be shown posts of that specified language; therefore, if you choose an uncommon language, you will see very little content (I’m assuming the most common language that is used on Lemmy, currently, is English). The warning, on its own, is rather nebulous, to me.
You can select multiple languages in the box. The “Undetermined” option is for all posts without a language set. Most people do not set a language on their posts. So unselecting it will hide most posts from you.
This might be desirable if you, say, only speak French and don’t want your feed clogged up with unmarked English-language posts. But it also means that if someone made a post in French but forgot to tag it, you won’t see that post either.
If you’re, say, bilingual English/Spanish, you would probably want Undetermined, English, and Spanish selected in the box.