[mod] replace searx.languages by searx.sxng_locales

With the language and region tags from the EngineTraitsMap the handling of
SearXNG's tags of languages and regions has been normalized and is no longer
a *mystery*.  The "languages" became "locales" that are supported by babel and
by this, the update_engine_traits.py can be simplified a lot.

Other code places can be simplified as well, but these simplifications
should (respectively can) only be done when none of the engines work with the
deprecated EngineTraits.supported_languages interface anymore.

This commit replaces searx.languages by searx.sxng_locales and fix the naming of
some names from "language" to "locale" (e.g. language_codes --> sxng_locales).

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>