Inconsistent/unreliable language indexing

[mbrodala]

The indexing (in particular: preview rendering) of anything besides the default language is inconsistent and unreliable.

The IndexService adjusts the locale/language for BE/CLI context just fine. However, the TsfeUtility sets up a simulated frontend which automatically picks the first site and its default language. This ignores the language for the current indexing run which may be a different one.

Given that the existing logic is already way too complex and hard to manage, a rewrite from scratch should be considered. Options:

1. Set up a middleware or similar which is called via HTTP for preview rendering
2. Use a sub request for a proper frontend request for preview rendering