I would say that the Crimean referendum, like the Chechen referendum, was not legitimate because it was carried out under Russian occupation. If no Russian forces had been present, and UN observers had observed the proceedings and given it the all clear, it would be legitimate.
Of course, if it was up to me I would have dissolved the little Russian statelet's autonomy long ago and made Crimea an independent state with Crimean Tatar sovereignty and special status as the native people guaranteed in the constitution (none of this Russian nonsense; Crimean Tatar should be the language of government) but I can't disagree with a strong democratic mandate, even one where the central government disagrees. My compatriots in Catalonia are experiencing that exact problem right now with their referendum. At least their country is unified; god knows when the Bretons will ever get that chance.
In the UK people expect immigrants to the country to learn and speak English, and to use English as the language of official matters. I think that's the same in most countries with their own respective languages; it's certainly the case in Russia. We don't expect immigrants to the country to stop speaking their native languages, however; they're welcome to teach it to their children and local schools should teach the language to strengthen ties between the various ethnic/national groups. Given that every Russian in Crimea is an "immigrant" or the descendents of immigrants, comparatively recent at that (i.e. within the last 100-200 years) why don't we expect the same of them?