Hogsnort pretty well says it all. Adapt your R/T to the local idiom. Best learned by listening to what the other users say. Leaving - vacating - passing - reaching and maintaining work well in the vertical mode as none of those words should confuse the foreigner. Left and right work best if restricted to the lateral mode for the same reason. Even the occasional parochial Aussie ATCO will understand these terms. ICAO is accepted here, therefore none of us have an obligation to use anything else. However, if local lingo does accept "left" as a reference to some vertical activity, there's no harm in that either. Me? I avoid the word unless relating to a turn but if my co-pilot wants to use it I am not about to get my knickers in a twist, as I know the locals understand it. However, if he does it on an international sector I will berate him and require him to buy beer.