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.
Yeah the UI is pretty bad at the moment. You can select multiple items by holding the Shift key and clicking them. I don’t know how to do it from mobile.