Set up translations for the frontend strings

[ahderman]

Context

Internationalization should be done early in a project, otherwise it becomes really difficult to add.
The library https://github.com/kazupon/vue-i18n is made for that and should be used.

Acceptance criteria

• The vue-18n library is added to the project and allows us to specify translated strings in the application as described in its documentation
• All hard-coded strings in the application are replaced with the equivalent $t() calls, and a file fr.json is created to contain the french-language localization strings.
• If needed, tests are adapted to ensure that they don't depend on hard-coded translated strings.