I work for an international company, and the working language is English. If you're on a meeting, you can expect it to be conducted in English. But it's normal to talk to customers in their preferred language where possible - not a shop floor, but business customers, so it's a bit different.
I think it's unreasonable to stop people talking whatever language they like in their break times, but if there is an issue with other colleagues feeling excluded, it's okay to take a stand on that. Language can be used to exclude people - not just foreign languages, but the specialised language of different disciplines and industries, and of also social groups, slang and so on. It can be a very powerful way of including or excluding people.
But I wouldn't want to ban different languages entirely. One of the things that keeps me working for my current employer is that I can walk down the corridor and hear English - and Polish, German, French, Italian, Spanish, Dutch, Indian languages, Asian languages. No idea what many of them are saying, but it makes life more colourful. People sometimes forget, and I have had to point out to Dutch colleagues that I'm not totally fluent (I've virtually no Dutch at all, but I can sometimes work out the gist of a sentence in context.) I love seeing the different way English gets phrased, when it's clearly influenced by German punctuation and German phrasing. Sometimes you see there are different ways of thinking about things because of language, and getting different perspectives are likely to help us get better solutions for problems.
The issue is whether languages are being used to include or exclude - if it's the latter, that's an problem and should be dealt with, but I am not sure a blanket ban is the way to go.