1. There was a trend a few years ago to get your language certificate for employment... That trend has largely disappeared nowadays, as far as I can tell. People won't ask for certificates, they'll just have you speak or write something in English to prove your ability. Almost all of my classmates from my masters program know English as a second or third language. They've all had interviews and no one has asked them for certificates...

2. I think the TOEFL is accepted by all schools, with the IELTS a close second (although from what I hear, the TOEFL is much easier than the IELTS).

3. The TOEFL and the GMAT have only one section in common: reading comprehension. This is much easier for the TOEFL than it is for the GMAT... While studying for the TOEFL might help a bit in your GMAT preparation, it's by no means enough.

Dear Dana,

thank you very much for your detailed answer.

TOEFL might really be the best solution of my problem. Though it is valid only 2 years, it might still be evidence for a life-time. And as you said - every interviewer would expect you to proof the certificate 1 on 1. Latter makes Cambridge certificates somehow redundant.

As for GMAT and TOEFL - they are certificates with a different target, but I have read several articles, stating thatn taking TOEFL prior to GMAT and receiving a score around 100 would ensure that you are fully capable of understanding the English, used in GMAT. TOEFL can also be a kind of refreshment.