(See http://luzme.uservoice.com/knowledgebase/articles/308153-how-do-i-change-my-settings- if you're not sure how)
Click on the "Languages" tab.
You'll see a list of languages. Select those you want, and uncheck any you don't. And then "Save".
Please note: this setting is not foolproof. If a store doesn't mention what language a book is in, then we will not know. But Luzme will filter out all books where we know it is in a language you don't want.