Add French translation and add django-rosetta for easier translation

[cro]

This PR adds a DeepL-generated French translation, adds some {% translate ... %} calls in the templates for strings that appeared to be missing, and fills in some of those strings in the Spanish translation.

It also adds django-rosetta to assist in future translations.

Apologies in advance, while I do speak some Spanish and French, I do not consider myself fluent in either language.

[cro]

Yes, I did accidentally commit my DeepL key 🤦 ...I already changed it.

[danihodovic]

I missed this. I will have a look at it next week.

[danihodovic]

Released as 0.3.0. Thank you.