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.
Most content does not have a language tag. Anything without a language tag is treated as if it has the “undetermined” tag. So if you have not listed “undetermined” amongst your list of languages, then you won’t see most content, because most of it isn’t tagged.
The most used? Yes. The most tagged? No.