As a developer I want SPI to only return the topmost language requested by user, so that system (sql transfer, cache size, php memory usage) can work more efficiently.

Currently the language parameters works as a filter, returning all languages user asked about, but as of EZP-27428 it starts to work more as a prioritized languages list. Which is what was more common in legacy, and is also the logic performed in Solr Search engine, and to some extent the LegacySearhcEngine.

Scope: v2 (This is a slight BC break when specifying language to API, however for 99% use cases it won't be detected as they already use logic on top, TranslationHelper, selecting top most language already anyway)