Mike,
Article 10 of the European Convention on Human Rights gives the right to freedom of expression and this is the most-used starting point when arguing for the right to use a certain language; should you/your union bring an action, it would most likely be under this head.
However it has not been fully tested by the European Court of Human Rights so it's still a grey area.
Article 10 does allow some derogation from the freedom:
The exercise of these freedoms, since it carries with it duties and responsibilities, may be subject to such formalities, conditions, restrictions or penalties as are prescribed by law and are necessary in a democratic society, in the interests of national security, territorial integrity or public safety...
- hence an employer such as an airline may be within their rights to stipulate a certain language on those grounds. CRM has been mentioned previously and if you have multi-national crew, it may be that the airline considers it best if they are used to dealing with each other in one language as a matter of course.
Likewise, they may actually be operating under a view of non-discrimination. That is, if you have multi-national crew and some start speaking to each other in a language which effectively excludes others, then that could be discriminatory (
). Hence they have to be seen to be doing something to control that. Whilst you're on a trip, you're on duty, even at the pub, so they get to say what goes. Proof of that can be seen in two simple examples: 1. if you fell over and broke your arm at the pub whilst on a slip, you could claim on the company's insurance; 2. if someone behaved inappropriately towards a female F/A, the company could go after the culprit. Hence whilst on a trip, you're on duty, even when you're not at the wheel.
Now you claim that your employers are making this rule so that they get to hear everything that is going on. But if you work for a Spanish (or any non-UK) airline, then how will it help them if everything is in English? Surely they would be better to stipulate everything outside actual operational conversation (eg. with ATC or checklists), be in Spanish?
Moreover, if you're complaining about them wanting to hear your conversations off the aircraft but whilst still on duty, then you are saying that they have ears EVERYWHERE - in which case I suggest you need to be careful of who you're talking to, irrespective of the language used.
I would say that this sort of stuff is gold to any 'popular' newspapers... I wonder what management would say if they found out that it was going to hit the press.
This article may be of interest.