Ps.zope.i18nfield

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- Add __hash__ method to I18NDict.

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- Add missing method definition for ILanguageAvailability interface.
- Add plone.registry persistent adapters.
- Add required Plone utilities.

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- Allow for indexing of normal dict and other values as well as I18NDict.
- Better to iterate over the existing indices in case the available language configuration changes.

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- Added en translation.

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- Added translations (de, es).
- Provide the 'Add translation' value already translated (e.g. for JS).

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- Return empty dict when value is None.
- Test for correct value types before we set any data.
- Correctly evaluate the bool value of the I18NDict when the field is required.
- When the fallback is the first value found, sort the values so that the result is consistent with each repeated call.