Multiple Source Languages in XLIFF Doc

Hello,

we are using German as the source for all our translations.
But some Translators are not native german speakers and would
see an additional source language, english or spanish for example.
In XLIFF only one source and target element is allowed per Unit.
How can we solve this problem.
With more source elements or with multiple XLIFF Files?

 Best regards,

Uli L輍eler