Is that becuase of the Laguage of the OS, or the installing users regional settings in Control Panel?
That depends:
If you install SQL Server running under the SYSTEM (machine) account, it will pick up the OS's regional settings (so if your OS is German, the regional settings for the SYSTEM account are German ...).
If you install SQL Server running under a dedicated account, it will use the regional settings of that particular user/account.
Regards,
Michael