There really is no such thing as an internationally valid licence. Each State is responsible for issuing its own licence (or equivqlent authorisation) for use in its own territory. Many States will accept a licence issued by another State as the basis for permitting the holder to provide ATC in their own territory (although often the States' licences that are recognised are rather limited).
If you can demonstrate that your training met the Standards in ICAO Annex 1 - which are very basic - and have experience providing services in accordance with PANS-ATM etc. you should have a good start when discussing with a State's CAA how to get one of their licences.
As niknak alluded to, the situation in the European Union is a bit different - although by no means clear. There are new rules due out any day which might give you a licence that can be used in any EU State - but the requirements for getting one are far more detailed than those in ICAO. That is not to say it is 'better' than a licence issued outside the EU, just that there are far more boxes that need to be ticked in order to qualify for the licence.