I've jumped on UAL several times and also on CAL and DAL; each time I was in uniform but changed to civvies later when on board.

When it comes to code sharing – remember that there's no such thing as jumpseating in Europe for their own pilots, in fact my friends have to buy tickets for themselves when riding on their own airline (SAS) although it's pretty cheap.

Having said that, I have gotten to 'jump' within Europe on Lufthansa twice now but each time it was because I lucked out and ended up talking to the crew at the check in counter (first time the FO who then talked to the captain on my behalf, second time the lead FA who did the same).

The captains then let me ride in the back and made some phone calls (to their dispatch I think?) - not sure what kind of ‘pass’ they used for me but both said it didn't cost them (or me) any money, otherwise I was going to repay them. Great service for sure, and I wrote nice thank you notes to the crews.

Not sure where you're heading but if my intuition is right and you're thinking about heading to PL check out Wizz Air - have flown on them a few times and I thought it was very cheap and the service was good too. Most of their pilots are either Hungarian, German or Polish so since you speak the language maybe you can get some special deals...

PS. They fly out of Cologne.