Language moves with the times, and some slang might arguably be better than current ICAO; for example; 'point' instead of 'dey-ci-mal' - the fewer the number of syllables, the better I reckon.
But the thing is; all of the ICAO compliant organisations need to agree such terms so that all pilots and ATC {who speak all sorts of different languages, not necessarily English} only use approved phrases, and all know what is meant by such terms. Slang that is not ambiguous for one person might be incomprehensible to another - resulting in at best extra read-backs, confusion and too much time taken up on busy frequencies, or at worst a flight safety incident.
So get them approved by ICAO and I'm sure we will all be happy to use them (and know what they mean).
Simples
