Taildragger
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
The European Convention on Human Rights isn't really relevant in this case. The conventions written at the Council of Europe are a set of legal obligations which member governments agree to adhere to, most of these obligations are enshrined in law in each member country. Ultimately, a case can go all the way here to Strasbourg (in fact we have 80,000 of them waiting to be heard) but it's important to understand that only a member government can be prosecuted in the European Court of Human Rights, it is not a place where an individual can attempt to prosecute a private corporation.
With regard to freedom of expression the point of the convention is more to do with your ability to say what you think without being harrassed by a government instrument, like the police for example. It has very little to do with language, there are various other conventions which cover protection of language though none of them cover what a private corporation may or may not impose on its workforce, that's an entirely different legal playing field.