To the best of my knowledge...
Luxembourg doesn't issue licenses, they just issue validations. So it's your problem to keep your license current (and therefore your employers' problem too). You're flying with an FAA ATPL ? Keep it current and your validation will be current. Now obviously, if you're FAA, it's not such a pain in the ass as with JAA.
But things will change and JAR-OPS/FCL/etc. should be fully implemented some day. So for the time being, any ICAO license is okay, but you better have a European license in your pocket too !
See what I mean ?