Items themselves aren’t tagged with a language internally in Omeka S, so there’s no such query you can do.
The individual values do have a language tag, and that’s included in the API output.
There is a “Language” Dublin Core property that a site could use to tag items as a whole with a language, but using it is totally optional. In the case that that’s been tagged consistently, you could use the normal property search query to limit items by the value in that Language property.