The problem I have only exists in TYPO3 6.2.x. In 7.6.x everything works as expected!
The steps are as follows:
The problem is, that some strings are not translated, but their key is displayed, e.g. like "LLL:EXT:my_ext/Resources/Private/Language/locallang.xlf:flexform.content.tab". A lot of other strings (in the same plugin) are being translated just fine.
What I have tried:
What I haven't tried yet, is debugging TYPO3 itself. I'd like to do that but I don't like to find the important classes to check. Can someone lead me to the parts in the typo code where debugging could be interesting?
After two hard days of work I finally found out. You won't believe it. The problem was in the flexform.
This will translate just fine in TYPO3 6.2.x :
<sheetTitle>LLL:EXT:myExt/Resources/Private/Language/locallang.xlf:my.key
</sheetTitle>
This won't:
<sheetTitle>
LLL:EXT:myExt/Resources/Private/Language/locallang.xlf:my.key
</sheetTitle>