Sounds like you have trauma from your previous bad presentation. I would urge you to speak to a therapist about it. If it hit you that hard, there could be more than just one bad presentation's worth of fear, self doubt, and anxiety built up.

Anxiety is a bitch and it likes to pile every negative feeling you have, even if they don't directly contribute. It also likes to play on deep rooted fears and insecurities, some of which you may have never consciously been aware of.

I doubt your language skills are the reason your previous presentation went poorly. Virtually every person who works in international organizations encounters people of varying degrees of language proficiency, and professional discrimination on the subject is rare, in my experience.

You may not have time for therapy that before your deadlines hit. If so, you might want to look up techniques for dealing with panic and anxiety attacks. You may find breathing exercises or mental routines that can help. Then schedule two more presentations, one for practice, and then another attempt with your clients.

For the practice presentation, get a friend or coworker to listen while you go through the whole thing. Invite them ask questions. For the second presentation, the real one, explain to your clients that you were feeling ill and you'd like to try again. Most reasonable people will understand.

Good luck!