The has been a lot of misunderstanding around full text search, stemming and diacritics in different languages. I find the documentation misleading and incomplete.
Using $language in a text search with a different value than the index's default_language can lead to very unexpected results, including not finding exact matches (event without diacritics).
In most cases you should not specify a different language than the default, unless you know exactly how the stemmers work for each languages used.
Incidentally, "default_language" is also misleading (but correctly documented) since it not only sets a default language for search queries but also defines once and for all how text is indexed.
A simple example:
A more thorough example: the value indexed is "passés". ** We search for various terms using different index/search languages. The outputs are quite unpredictable except for the fremch/french and none/none cases:
|index default_language||search $language||"pass"**||"passe"||"passé"||"passes"||"passés"||"passées"|
Moreover, in this page, The given example has no value since it does not use stemming nor diacritics, and would yield the same result with no $language specified.