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.
Just make sure that Unspecified is selected as a language in your settings, in addition to whatever languages you can read. Don’t un-select “unspecified.”