I do understand what you are asking for and I agree it would be a nice feature. A easily set search filter that will match streamers with a specific language set would be handy. Forcing the streamers set the languages they use in the stream would be nice for viewers in general.
Until then:
Have you tried adding the language 3 letter abbreviation to your search? For example: [ENG]
Not every stream sets the primary language for the stream, but if they do, it’s searchable.