Civil aviation law is the point. Not ICAO recommendation or practice.
In my Civil Aviation law is very clearly written that on frequency you may speak English or any other state' official language( We have officially three by name ,but it is one actually) . So we have to.We are aware that it might rise some safety issue but we solved it with adequate sentence in MATS. If any doubt could arise from using different language on frequnce than operational ATCO
must translate to all concerned parties all necessary information.
De facto situation is that 99 % of pilots use English language,at "resonable" level,and only a few military pilots or sport pilots croosing control zone sometimes use "local" terms and procedures , widely known as "shall we...."
but you have to leave the legal space for use of local language because of the constitution matters. Everyone has right to speak his own language.
Since the constitution is the highest than all lower level documents must follow constitution.