Reservations in Germany are only possible in long distance trains (IC, EC, ICE) and are optional. Unless a train is massively overcrowded, you'll be able to board. Reservations are cheap though, €3 per train on tickets.oebb.at or €4.90 per journey (for all trains in total) on bahn.com. Don't book via Interrail.

Your pass is your ticket, so you don't need to book any other tickets. Seat reservations can be booked until trains are full. The TGV/ICEs between France and Germany are very popular, so it's best to book them as soon as you know your travel dates. You can book them at an SNCF ticket office free of charge; ask for a parcours/traject d’approche. The same is true for any domestic TGVs in France.

Do you live in France or Germany ? Your profile indicates France.

Seat reservations are optional in Germany. You should always find a seat but sometimes you may have to swap seats or stand for part of the journey. Board at the end of the train (always less busy). If you'd like to make a reservation, do not use interrail.eu (way too expensive) :

- 3€ per train on ÖBB

- 4.90€ per journey on DB

International trains have mandatory reservations and you need to book them well in advance (especially for 12th August) : 17€. You can avoid them by taking a regional train across the border : Offenburg/Appenweier - Strasbourg, Saarbrücken - Forbach, Lauterbourg,…

Seat reservations are mandatory on TGV and Intercités trains in France : 10€ limited fare, then 20€. Some routes are very popular so book well in advance too. Use https://travel.b-europe.com/Eurail-GE/en/booking-tgv#TravelWish to save booking fees. Free if you're a French resident as advised.