If all pilots on a frequency could understand all communications?
They don't. At least pay attention. Ever since this argument came up, I have watched my captains, later my copilots and the majority do not pay much attention to transmissions that do not start with their callsign and trigger their attention ("cocktail party" effect) in English-only environments. I have seen this on both sides of the Atlantic. To summarize:
Do you think safety would be:
* No different
No different if you look at Europe and Canada. Worse if you consider Latin America.
it appears that the preservation of their language is a higher priority than flight safety.
that depends. In the US, Southwest usually gets priority handling, in Germany LH or AB, in France AF, in the UK (depending what airport) UK carriers over foreign carriers. I have seen it all.
It is more an issue about the quality of an ATCO (or system) than the languages used. As ATC Watcher has pointed out correctly, no dual language or non-English transmission has directly lead to an accident. With every accident there are always lots of ifs and it is easy to blame it on a foreign language that one doesn't understand. Let's not start talking about UHF traffic that we cannot hear, that fact usually gets quietly ignored.
