Go n-éirí an bóthar leat / Go dté tú slán(singular)
Go n-éirí an bóthar libh / Go dté sibh slán(plural)
Have a nice trip! = Miłej podróży!
Have a nice trip = Bon voyage! (Not sure what you mean by "When you are flight".
¡Buen viaje! (Have a nice trip!)
bon voyage!
Buon viaggio.
Slán agus beannacht leat.
Lá breá a bhí ann.
Golgu todu show du
In Croatian, "Have a nice trip" is expressed as "Sretan put." This phrase can be used to wish someone a pleasant journey when they are about to travel.
Go deas bualadh leat.
Tá folt breá ort
In West Galway you could say "Tá áthas orm casadh leat".