I've jumped to a project where I'm developing a private site. The site has been using faceted search as a search engine. There's a keywords search and guided search. Guided search includes "content types", "language" etc. Now what I'm supposed to do is to add there search options like for example [language] + [content type] which would search and list all the items from specific content type with specific language.

The problem is that I don't know where to start and which module I should start customizing? I tried to check the documentation, but didn't find any starting point.