My suggestions would be:
Make sure that you use the standard phraseology. Buy CAP 413. Each word has a specific meaning, so it is important to use them correctly. For example; the difference between 'Roger' (=I have received and understood your transmission) and 'Affirm' (=Yes). It might seem petty, but it's to avoid confusion - and also so that non English speakers can understand and be understood.
Don't try to speak quickly.
Bear in mind that when talking to a non English ATC; if you speak too quickly or use non standard phrases, you may not be understood at all.
Don't try to be flash.
Don't copy what you hear or what they say in films ('Roger that'...'This time'....'Go ahead'....etc).
Good luck!
U