Hey,
It mainly depends on what languages a certain station is released for.
This is clarified in the AIP for each country.
To give you an example:
Wien Radar (the enroute control) is only permitted to communicate in English
The same for any APP positions around airports (Graz, Linz, Innsbruck Radar)
BUT
The tower positions in Austria are permitted to speak German OR English, depending on the initial call of the pilot. (He may choose which language to use - the controller then has to reply using the same language)
One reason is, that VFR traffic often is easier to handle in your "mother tounge" - if there is more info to give and of course this is a relict of the good old times. Nowadays more and more pilots primarily use English. Radar-positions have to be handled in English, as any kind of traffic may occur (Chinese pilots, German pilots, Spanish pilots ....)