In case anyone is still struggling with this problem as I was, here is the solution.
Create different Business Zones for each language that you require Setup -> General -> Time and Place. Use the Country dropdown to select not only the locale formats, but also the language you want to define an E-Mail template for
Set the Location to use the correct Business Zone Company -> Locations & Rooms -> Business Zone.
Append a new language to the E-Mail template E-Mail -> Templates -> Template. In Other Languages use the Languages dropdown and select the same country as defined in the previous Business Zone
Define the new language template.
Now when an E-Mail is generated for a client having a location with a specific Business Zone, the corresponding language template will be used.