Add Language and Site serializers

[tvdeyen]

What is this pull request for?

Without those ActiveModelSerializers can't find a serializer for the site and language relations in the PageSerializer which is used in the PageSelect. It delegates serialization to Rails which takes whatever serializer is responsible for to_json, which in some cases lead to nesting the site or language with it's own root key, which is not what we want.

Checklist

• [x] I have followed Pull Request guidelines
• [x] I have added a detailed description into each commit message
• [x] I have added tests to cover this change

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 95.96%. Comparing base (98293af) to head (1447087).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2900 +/- ## ======================================= Coverage 95.96% 95.96% ======================================= Files 230 232 +2 Lines 6269 6275 +6 ======================================= + Hits 6016 6022 +6 Misses 253 253 ```

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