Make some button's wording translatable

[Jerome-Herbinet]

Please make some wordings translatable ; thanks !

Please find below a French screenshots.

Can you complete translation files ?

[beleon]

The button labels ("Upload", "Cancel") are already wrapped in translation calls, they'll be available for translators once the Transifex sync is re-enabled, which is in progress (nextcloud/docker-ci#927).

The error in your last screenshot is actually a bug in transfer, thanks for making me aware! I have a fix ready and will put out a new release tomorrow.