Book creators uploading non-English books are asked to give the language name in the title line so you should be able to use the search function and insert the specific language you are looking for.
As far as I am aware not all or better only a few uploaders tag their books with a language tag. So you can use tags for searching but you will only find a part of all books available.
A table of all German books available at MobileRead you find here:
https://wiki.mobileread.com/wiki/Free_eBooks-de/de