Improve documentation of `addcorpus.models.Field.language`

[BeritJanssen]

The language field of the Field model looks strange to me: It has a CharField with blank=True, null=False. So then we should set a default, I believe? NB null=False is not necessary to state explicitly (and it's not advised to set this to True on CharFields anyway). validate_field_language seems to assume that fields which don't have an explicit language tag set would have to have "dynamic" set. I'm not sure I agree with enforcing that every field, or for "dynamic", the corpus, has a valid language tag, in some corpora we simply don't have that kind of metadata. A docstring in validate_field_language about the meaning of the "dynamic" setting may also not be misplaced.

[lukavdplas]

• You're right that null=False is also the default value, so this isn't really needed.
• For CharFields (at least with null = False and blank = True), '' is already the default, so you don't need to set it explicitly.
• validate_field_language also accepts '', but the logic is a bit odd because it calls validate_language_code which is also used elsewhere. I agree a docstring would be a good addition.
• Possible values are explained in documentation
• I don't think a validation function is the best place to explain how values are implemented, but the docstring of FieldDefinition would be an appropriate place for more explanation.

[BeritJanssen]

• For CharFields (at least with null = False and blank = True), '' is already the default, so you don't need to set it explicitly.

Ah! So that's indeed a default. Hadn't stumbled upon that yet in the Django documentation. I do think that if we are to explicitly state values, blank=True, default='' is more readable.