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.
There are posts older than the language option. Those posts don’t have a language set and ate therefore undetermined. You won’t see those posts if you deselect this option.