Why?
For example, if you work in Sweden, you would need to know Swedish (level 4 I think the requirement is).
Swedish is to my knowledge not an ICAO language, but it is approved for communications in Sweden. There is even a Swedish spelling-alphabet (ie a Swedish version of Alpha, Bravo, Charlie etc) to be used in Swedish comms.
Also coordination would be done in Swedish, local procedures written in Swedish. People calling on the external phone line (for those regional towers where you might quite often be the only person there, no supervisor or admin staff to take the phone for you) will expect you to be able to have a conversation in Swedish.
There are ICAO languages, but there is also the society within the air traffic control unit functions. Quite often this requires the controllers to know the local language, Maastricht is one of the few excemptions I can think off where you would not have to know the local language to work. I have a faint memory of reading that Rein have recruited non-german speaking controllers too, so I'm assuming all coordination has to be done in English then.