[Feature] add language variant fallback support

[tanyunshi]

Some one requires translations in English(United States) also understands British English.

For example, if we don't have translations for en-us, we shall translate the object in en-gb if the translation exists. For example,

>> f = Fruit.objects.get(name='苹果', translations=
        {'de_de': {'name': 'Apfel'},
         'tr_tr': {'name': 'elma'},
         'en_gb': {'name': 'apple'},}
      )
>> f.language('en_us')
'apple'
>> f.language('en-us')
'apple'

[tatterdemalion]

It would be a nice feature. I am trying to think of the ways how we can achieve this.

There are multiple ways to achieve this. But I would prefer to be able to do it with a single query.

We can always search keys starting with languagecode_othercountrycode but this has some disadvantages.

• It would be painfully slow for large queries
• For instance zh_cn and zh_tw are basically two different writing styles for Chinese. They are not really interchangeable.

We can make another settings like settings.TRANSLATIONS_MAP which contains a list of strings containing interchangeable language keys:

TRANSLATIONS_INTERCHANGEABLE_KEYS = {
    'en_us': ['en_gb', 'en_au'],
    'en_gb': ['en_us', 'en_au'],
    ...
}

This way we can always do a has_any query and retrieve one of the interchangeable translations. That's the most efficient solution I can think of.

[Natim]

@tatterdemalion would you have time to review the related pull-request?