Fix translations

[audez]

Context

Some strings are not translated in the UI. It comes from the fact that:

• some strings are not wrapped in the translation function
• in some files, the strings are wrapped in the translation function - and most of the time they have been translated in Weblate - but the translation function has the wrong scope, leading to a non-translated string on the UI

Proposed solution

Here I fixed some "simple" cases that I can handle, specifically in the "form" module. For more complex cases, I let @fflorent and other good developers take the lead :)
I will do more PRs for other strings.

The solution is:

• Wrapping the strings in the translation function
  and/or
• adding the right scope to the translation function

Related issues

#627
#962

Has this been tested?

• 👍 yes, I added tests to the test suite
• 💭 no, because this PR is a draft and still needs work
• 🙅 no, because this is not relevant here
• 🙋 no, because I need help

Screenshots / Screencasts

An example of untranslated strings on the French UI:

[fflorent]

LGTM. Thank you @audez! 🙏

[paulfitz]

Thanks @audez, @fflorent