Add French translations

[annavik]

In this PR we add French translations (thanks @Annie-Shan ! ⭐ ). We also add a language control to make it possible for anyone to switch language.

Summary of logic

• We will use the browser language as the default preference, with English as the fallback value (when the browser language cannot be matched to English or French).
• If the user changes language from top bar, we will override the browser language and store the preference in a cookie. This means next time we can present the website in the preferred language from start.

Notes

• Do you think we should translate section "Academic Publications"? The resources we link to in this sections are all English, but we could perhaps present translated summaries?
• Do you think we should translate the 11 items under "The AMI Initiatives"?

Screenshot

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