[Accessibility]Enhance accessibility

This application must comply to the RGAA https://www.numerique.gouv.fr/publications/rgaa-accessibilite/methode/criteres/#contenu
To comply a few enhancement must be made and checked during all the development. Here are some issues we can fix :
* The language label of the page is set to "en" while the text is in French, we must have consistent label and language so that a text reader read the website in the accurate language.
* some links aren't focusable (like the breadcrumb, "Notre API", "Nous contacter") so a person using its keyboard to navigate can't use them
* forms must be labelled (the form that serves to choose the realm is not for now) otherwise people using screen reader don't know what they have to write in it
* some text slightly lacks of contrast (the middle image text and the dark theme header)
* a sitemap can be useful as an alternative to the breadcrumb

Start by reviewing the linked RGAA criteria and auditing the French page language, breadcrumb and other links, the realm-selection form, text contrast, and navigation alternatives. Done means the listed accessibility issues are addressed and checked against the RGAA guidance.

Written by the indexing model from the issue text.