Add product field "lc" as "product main language"

[monsieurtanuki]

Why - Problem description

lc exists as a product field on the server side.
We don't have it in off-dart.

What - Proposed solution

We should add it to the product fields, probably with a more explicit name like mainLanguage.

Alternatives you've considered

I don't know how important that field is, and it may cause confusion with the product query parameter lc.
Therefore we may not add this field eventually (low interest + average confusion risk).

[monsieurtanuki]

cf. openfoodfacts/smooth-app#4746 (comment)

Not relevant in the end to add lc as a product field.
Just note that lang is a read/write product field that contains the product main language.