Hi Loulou, 
Please allow me to answer in English. 

1.Your reservation is validated as soon as you paid the fee and received your confirmation. Reservations are not meant to be added to the app. You just take the seat reservation with you and show it next to your Pass when your ticket is being checked. 

2. You'll have to pay for your reservations upon booking. You can book your reservations beforehand (this is with some trains recommended) or at the train station. 

3. When your reservations have been paid for, the refund policy of the railway carrier applies. For more information on this have a look on this page (and scroll down). Whether it is modifiable and/or can be refunded depends on the carrier. In appendix 1 of this page, you can find some links to the policies.  

4. That's correct. As an alternative you can book your reservation at the train station. If you can share the details of the route you are planning to take (destinations, date, time), we can suggest other alternatives if there are any. If a reservation is required for the train, you need to purchase a reservation. Adding it as a journey to the mobile Pass only is not enough. 

5. This means our App is not able to retrieve how many seats are available. This is often because the railway carrier did not provide this information. When adding it to your cart and continuing to check-out will show you if there is a seat available for you to reserve. 

6. Did I understand it correctly that you want to know how to have seats next to each other, when you have both passes on different phones? When making reservations you can edit your '’trips& travelers'’ in your account. For instructions have a look here. If your trains do not require reservations you can both just hop on and take seats next to each other. 

I trust this clarifies it. Cheers, 

Merci beaucoup !!

1. D'accord, et donc comment cela se passe pour les trajets sans frais de réservation ? (pour que le trajet soit réservé)

4. J'ai vu que l'on pouvait réserver directement sur le site des compagnies ferroviaires, et dans ce cas comment faire pour ''payer'' avec le pass ? Y-a-t-il un onglet dans chacun des sites où on peut réserver le billet grâce au pass ?

Merci encore pour vos réponses !

1. For trains without mandatory reservations (which is the vast majority of trains), you don't have to book or reserve anything. You just add the train (or a journey with multiple trains) to My Trip and then to My Pass. Do you know the mobile pass FAQ?

4. The method depends on the train company. Some require you to indicate you have Interrail/Eurail/a Rail Pass, sometimes with a “pass cover number” (you can request one here). With others, you can just choose "Seat only” or "Reservation only”. See this topic for some examples: 

    

Merci beaucoup pour vos réponses ! Malheureusement, j’ai encore des questions…

8. Pour les trains qui nécessitent une réservation où l'on doit payer, je pense avoir compris, mais pour ceux qui ne nécessitent aucune réservation/aucun frais, comment pouvons nous recevoir un E-Billet ou code barre ? 

9. Comment fait-on pour ajouter un train manuellement sur l'appli Rail planner ? (exemple un trajet Strasbourg-Metz indisponible sur l’application)

10. Voici l'itinéraire que nous comptons faire, les trajets en violet (et le orange) étant ceux qui nous posent problème, et dont ne savons pas comment en faire la réservation. 

Pour le Metz-Mulhouse nous comptons nous renseigner directement à la gare. 

Pour le Ljubljana Zagreb, nous pensons avoir trouvé le trajet correspondant sur le site OBB, on a réussi à trouver l'endroit pour mettre la réduction interrail, mais nous ne savons pas comment aboutir à la réservation d'un billet. (on a bien 0€ qui s'affiche mais nous n'arrivons pas à aller jusqu'au bout.)

Pour les deux autres en violet, nous ne savons pas par quelle compagnie passer... 

Enfin pour le orange, on a aussi un problème car on ne sait pas trop à quoi cela correspond et quelle est la différence avec le violet ?  

Et pour ce qu'il s'agit du trajet 2, auriez vous une autre alternative qui serait plus courte ? Car devoir remonter jusqu’à Munich nous paraît peu cohérent géographiquement…

Merci encore pour votre aide !

8. See my answer to question 1. You first look up the journey in the planner, then add it to My Trip and then you add it to your pass. After that, you will have a barcode under My Pass to show. You need to do this for all trains by the way.
9. At the bottom of the planner results, there is always a blue link "Add it manually”.
10. It's still too early for most reservations:
    Metz - Mulhouse: the best is indeed to buy at at a French ticket office. That's also cheaper (10 €) than from Interrail online (12 €). Not now, because it's still too early (normally 3 months in advance).
    Mulhouse - Zürich: is possible from Interrail online (27 €), but is cheaper at the ticket office (25 €). Still, that's a lot of money for just the 20 minutes until Basel: in Switzerland, no reservation is required and almost nobody has one. I'd take the TER to Basel and then IC to Zürich. No reservation necessary, you save 25 €. The TER timetable from 22 July is not yet available, but it should be Mulhouse 10:16 - Basel SBB 10:40/11:06 - Zürich HB 12:00 if I check earlier dates.
    Zürich - München: reservation possible via DB (choose "Seat only”, 4 €) but not mandatory.
    München - Rijeka: reservation possible via ÖBB with these steps (14 € for a seat) but not yet. Try again from 3 months in advance. Travel via München seems to be the quickest way. The night train also stops at Rosenheim (0:03) and Salzburg (1:40) but that would require an extra travel day.
    Rijeka - Ljubljana: reservation is optional. It's too early to make it (3 months in advance). Do that in Rijeka at the ticket office when you arrive there or online via ÖBB (choose "Seat only”, for 3 €). That's cheaper than via Interrail (6 €).
    Ljubljana - Zagreb: departure is 14:45. Reservation is optional, not available online. If you want one, do it at the ticket office in Ljubljana when you arrive there.
    Zagreb - Wien: Reservation is optional, can be done online via ÖBB (choose "Seat only”, for 3 €). Interrail want 8 € p.p. + 9 € delivery costs…
    Wien - München: Reservation is optional, can be done online via ÖBB (choose "Seat only”, for 3 €). Interrail want 8 €.
    München - Metz: you're only seeing very expensive and non-optimal options because the French TER timetable is not available yet. I'd expect something like this:

München - Stuttgart: reservation optional, 4 € via DB. Stuttgart - Strasbourg: 13 € from French ticket office or 15 € via Interrail.
Alternatively, you could do something like this, for which a reservation is unnecessary:

To be checked when SNCF have published their TER timetables from 22 July.

Merci beaucoup je vais prendre le temps de regarder tout ça !