Localization: Handle validation error messages

[MichaelKohler]

Currently the exact validation error messages are coming from the backend. These should be localized too. We need to figure out a reasonable way to do this.

[MichaelKohler]

After thinking about this, I don't think translating the validation error messages into all languages makes sense. Some of these are very language specific.

On the other hand, we are using the EN validation file as a fallback, so we'd expose English strings for those. Needs more exploration.

[MichaelKohler]

After exploring this for a bit, I think the approach here could be the following:

• We do not use the EN file as a fallback but rather a copy of it as default.js
• We document this properly in the README
• When a given language does not have a specific file, we use default.js
• For language specific validations we keep the hard-coded error messages
• For the default error messages we return string IDs to be translated
• We add those IDs as normal translations
• When rendering the component we check if it's a string ID and if so we correctly use the translation for it

While this is not perfect, I feel this is a reasonable way forward without having to implement the full localization infra on the backend.

[MichaelKohler]

:tada: This issue has been resolved in version 2.13.1 :tada:

The release is available on GitHub release

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