Problem located in my case in an Extbase extension because Extbase relies as standard Frontend rendering on config.language_alt to define an alternative language key.
We first have to revert #39220 I'd say because fall back mechanism introduced in 4.6 does not work anymore, it always falls back to English (= source).
By reverting though, we have the bug again in Frontend that a missing translation results in blank output, seems even when using config.language_alt (at least with Extbase).
But then, this config.language_alt is not an option because by reverting the patch, we are forced to set this to "en" in order to tackle with partially translated languages.
BTW, this does not seem to work with Extbase which seems not to properly fall back to the other language.
TYPO3 4.6 introduced a multiple-level fallback which cannot be handled by config.language_alt. I would expect the standard fallback mechanism introduced by TYPO3 4.6 to be automatically taken into account. The question is how? Because then config.alt_language does not make sense anymore.
Updated by 
Updated by 
Updated by 
Updated by