How strictly are language rules adhered to between Pilot and ATC?
Standard Phraseology is what should be used, but especially in those countries where the local language is an allowed IFR language, you will get deviation from that in daily use, either due to dialect, accent or simply colloquial terms influencing the phraseology used between pilots and ATC.

In countries like the USA, where English is the local language anyway, you will sometimes hear non-standard phraseology being used. In countries like Germany, where English is not the local language and IFR is only allowed in English, you will have pilots and ATC adhering to standard phraseology more closely, especially to avoid misunderstanding due to local accent.



Anecdotal comment: I fly in Germany and usually use English also for VFR. Germans have a very phonetically hard sounding pronunciation in English, making the language sound clipped and sharp, which carries over to phraseology being used. Many native English speakers have commented though that this makes it really easy to understand, because every word is spoken very distinguishedly. Although I speak English fluently without accent in normal day use, I tend to slip into a German accented English in aviation... because it's easier to understand.