[openstreetmap/openstreetmap-website] Add locale selector (PR #5201)

[David Tsiklauri]

This PR was updated according to the suggestion of @AntonKhorev.

As described in the `CONTRIBUTING.md` only updated language config was `en`:
> ## i18n
>
> If you make a change that involve the locale files (in `config/locales`) then please only submit changes to the `en.yml` file. The other files are updated via [Translatewiki](https://translatewiki.net/wiki/Translating:OpenStreetMap) and should not be included in your pull request.

Because of this, currently only English is shown in the list of language selector. At first, I thought to make a list of all language names in the config file, but when translations will be made, there will be a lot of duplication. The only downside of the current solution is that there will be only those languages shown, which have `shared.language_selector.language` key.

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