In my (non-airline) experience cross-cultural company marriages always underestimate the tribal loyalties (its bad enough when two companies in the same country merge TBH)
If everyone goes around still carrying a French or Netherlands flag tattooed on their metaphorical foreheads you'll get nowhere. You have to see it as an opportunity - and the best way is to mix up the workforce as fast as possible IMHO
Not very easy in rigid airline staffing structures