I am supposed to travel with Transavia un a few weeks. However, I won't be able to take the outbound flight. (I will take a plane the day after.)
Do you know if Transavia will cancel my inbound flight if I don't check in for the outbound flight?
Transavia is a typical low-cost airlines. To my knowledge, it only sells point-to-point tickets and does not have any complicated fares so you should be safe. I did not find anything about return flights in their conditions of carriage either (all the usual stuff about using different legs or "coupons" in the right order and allowing airlines to charge you or deny transport if you don't is absent). You might have booked the two flights at the same time but those are just two unrelated tickets in one web "shopping cart" basically, not a single ticket in the usual sense.
In any case, you can also simply call Transavia to cancel the ticket you are not going to use (it has to be by phone, it's not possible online, and they charge you for the call). You won't get your money back (I think you can ask to get some taxes back but not what you paid for the ticket itself) but you will have some peace of mind.
I messaged Transavia via Twitter messages and they were very prompt to reply:
Me: Hi! If I have a return booking, when skipping the outbound flight can I still use the inbound flight?
Them: Hi [name]
No problem at all !
If you wish to cancel a flight, let me know, I can refund you the taxes if there are some. :-)
Have a nice day !
So you can take the inbound flight even if you skipped the outbound one and might even get taxes refunded if you cancel the outbound leg of your trip.