Except for some low level functions, TYPO3 exclusively uses localizable strings for all labels displayed in the backend. This means that the whole user interface may be translated. The encoding is strictly UTF-8.
The default language is English, and the Core ships only with such labels (and so should extensions).
All labels are stored in XLIFF format, generally located in the
Resources/Private/Language folder of an extension (old locations
may still be found in some places).
The format, TYPO3 specific details and managing interfaces of XLIFF are outlined in detail in this chapter.