Thanks so far. We found an api call that delivers the installation language according to ISO 639-3 (that is, what is actually needed - not only the localization of the serial number which can be different)
The language codes according to ISO 639-3 are:
'DEU',
'ENG',
'SPA',
'FRA',
'ITA',
'NLD',
'POL',
'POR'