[TYPO3-english] TYPO3 showing language keys in backend instead of translation target

[Flo]

The problem I have only exists in TYPO3 6.2.x. In 7.6.x everything works as expected!

The steps are as follows:

    1. Fresh TYPO3 6.2.26 installation.
    2. Change language to german.
    3. Install extension.
    4. Create plugin instance.

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:

    - Deleting caches through using the flash / the install tool.
    - Deleting the Cache folder in typo3temp / deleting the complete temp folder.
    - Validating the xlf files -> everything okay
    - Check for differences in the non- and working translations like special chars, wrong keys and so on.

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?