We are moving again in the old language/cultural debate ( also open in the French ATC thread).
A correction :
We had the Shorts 360 fatal at CDG some years ago when language played a key part
No it did not, read again the report : a badly issued ATC conditional clearance ,combined with a poorly understood intersection line up caused this collision. (For memory : The T/O to the MD80 was in French but well before the conditional line up clearance issued to the Short, and both captains reported that none was monitoring the calls made on the R/T to other a/c due heavy workload at the time )
But there is indeed a recommendation in the report to study the use a single language in runway ops in CDG , but it was not a cause , even less a "key cause" .
In fact and this is "fortunately" or "unfortunately" ( I leave to you to choose the correct adjective ) there is no recorded accident where using a local language in addition to English was an issue.
Do not come with rumors and urban tales,( like Zagreb etc..) the fact is there is nothing in the ICAO accident database. This fact was even used by the Canadians to allow French in ATC in Quebec in 1976, and it still is in force today.
Cultural differences and lack of understanding each other are the problem, not the dual language.
Declaring an emergency for instance is seen in many cultures as an admission of failure. That has killed a lot of people. Dual use of language none.
Therefore ICAO has ( correctly and wisely in my opinion ) opted for the mandatory proficiency language training ( the famous level "4" ) instead of banning dual languages use.
For the Spanish or French ATC, yes they are very different from the UK, but who says the UK way of working is the one that everyone else in the world should use ?