Antoine I had this problem with Manjaro. My keyboard was always in us layout when I typed the passphrase. I didn't find how to change the layout at that moment. So my small advice for all Western people (latin language) on Linux is:
- Buy a US keyboard
- Use this keyboard layout -> English (US, intl., with dead keys)
You can type in French, Spanish, Italian, German, etc... The shortcuts on this picture are wrong (I often have to replace CTRL by Alt Gr).