Fix EZP-21125: (PAPI) Created language is only shown in admin interface after creating a new language

This PR resolves issue https://jira.ez.no/browse/EZP-21125

Created language was not visible in Legacy admin interface because of language cache not being cleared.

This implements slots to clear LS language cache on creating/deleting/updating languages through PAPI.
Tested manually.

This will not fix the situation where PAPI is used in CLI scripts and custom `FileSettings/CacheDir` is configured.
In this case language cache will have to be cleared by deleting the cache file directly on the filesystem.

Additional: fixed SPI cache issue where `NULL` return value of `LanguageHandler::update` was set to cache.