False friends
 

I heard an interesting linguistic issue on a Langen sector on Wednesday morning: the controller said "When you can be level xxx by yyy, descend level xxx". Since the aircraft in question was heading towards yyy at great speed, "when" didn't make much sense in that context, and the crew were consequently confused by the instruction, which was repeated unchanged.

This must be a difficult one for native German speakers: "if" in English is "wenn" in German. Any other examples of linguistic "false friends" in an ATC context?

Yes, even in English : the dreadful " Two" and "to" .
This lead to many accidents ( the most famous is the Flying Tiger 747 in KL in 1989) which resulted in a major change in phraseology to remove all the ambiguities .

In English some use " Oh " to say " Zero" " If french "au " ( pronounced " Oh" ) means " to " ( Cap au deux cent cinq = Fly Heading 205 ) so a possible false friend as you call it . There are much more.
For me ,sticking to standard phraseology, especially when not using your mother tongue is essential.

I'd say in the UK this also applies!

Abosutely Alfaman, think also of the US , but then they do not use ICAO standard phraseo but the FAA one , plus some "local" niceties.:E

When ready descend to reach xxx by yyy

In the US, "Cross yyy at and maintain xxx".