If you are on 2018.1 already you might try %SiteProperties.ApplicationLanguage%.
This variable does NOT work in previous versions but might do so on 2018.1. - there has been a change concerning the DSMC language.
(Don't know if the variable should work in 2018.1 and didn't try it.)
In older versions you might read HKLM\SOFTWARE\WOW6432Node\NetSupport\NetInstall\SiteProperties - NcpApplicationLanguage.
The value is set to 0 for german and 1 for english (don't know the other languages' codes).