Site theme UI translations

[thibaudcolas]

Problem statement

Follow-up to #447. We want to make sure all of the labels on the page are translated, not just the site content.

Proposed solution

• Review HTML templates and Python code to make sure all labels are translateable using Django’s gettext or translate tags
• Produce new .po files with all translate-able strings
• Use a tool like yesglot for a first draft of translated labels
• Update contributor documentation so we do this well in the future.

Alternative solutions

No response

Additional context

No response

Working on this

Assigning to @Raghaddahi