No sending possible after translation with WMPL

So, we use WMPL and Contact Form 7.
The original site is in German.
As we copied the form and put it on a site of which we changed the language from German to Russian, it was no longer possible to submit the form. The grey “waiting-arrows” are rotating and rotating and rotating. If we change the language into German again, it succeeds.

Any idea why? Thanx alot!