Streamline: European Labour courts (sic) do not exist. Employment issues, together will all other EU matters, are dealt with by the ECJ in Luxemburg. The Bosman case to which you refer concerned a narrow exception to the rule banning discrimination on the grounds of nationality where national sports teams were concerned (football, in Bosman’s case). This has no conceivable relevance to any aspect of the airline industry.
I know of no disputes concerning the airline seniority system at present. I have no doubt that the industry seniority system will be challenged at some stage in the future, but the challenge will (in my opinion) be based on sex discrimination principles, rather than on those of nationality. Systems where the level of remuneration and/or promotion are based upon length of service have been the subject of challenges before the ECJ in the past. I see no reason why ours should be any different.