ICAO Annex 10 Volume II Communications Procedures
ICAO PANS-ATM Doc. 4444 - chapter Phraseologies
In my experience confusion begins when people attempt to use language outside of the English RT phrases. For a long time, and becoming an unlikeable purist, it seems to me that you do not really need any more than included in the above. I.e. instead of thinking how to say, think what to say first.
When I started internationally we had a saying in our class that all is OK and we normally manage mostly ok, as long as the ATC and other traffic restrain themselves from using the local language. And the biggest headache was not the Spanish, neither French. Anybody care to guess?