wbsearchentities has a "profile" parameter, which only has two valid values on wikidata.org: "default" and "language". The default profile enables the regular prefix search functionality that works for all available entity types, while "language" is meant only for items to enable search for items representing languages. Language search was a feature specifically created for the Special:NewLexeme page. There don't seem to be plans to support other types of search profiles.

To have a generically named parameter that only enables one very specific bit of functionality, only in combination with one value of another parameter is strange from an API design perspective and also results in overly generic code. We should consider splitting it out of wbsearchentities and move it into a dedicated Action API module or REST API endpoint.