Hello Mickael M.
You can save a permanent template by editing the initial invoice file. You can find Invoice.fr3 file here: C:\Users\demyanov.EMAGICONE\AppData\Local\eMagicOne Store Manager for PrestaShop\Reports
Currently the Invoice template can be generated in one language only, however you can submit it as a feature here:
http://support.emagicone.com/823795-PrestaShop-Manager-Feature-Request