Future: add method to customize labels of translation and fallback fields

[danielvdp]

At the moment labels of fields are hard coded to include the language code and translation / default language in brackets. By including a customizable translatable string in Meta options this can become customizable.

[danielvdp]

An additional issue is that in a given project, makemessage will not automatically import translatable strings from external packages (apps), so it may be necessary anyway to define customizable labels so that translations become easier to include.