[meta] Make Commerce fully translatable

Fully translating a D7 site (interface, nodes, entities, etc) is really hard, and requires various additional modules (i18n, entity_translation, title, etc).
Hence, it is also really hard to fully translate a Commerce site.
In the previous year, various dependencies (entity_translation, title) have become more stable, and Commerce has been fixing internationalization bugs and adding / improving various integration (i18n, entity_translation) with every single release.

It's time to finish the job, and make sure there are no issues around this.
This meta issue is here to track all related issues.

Patches to supporting modules

Commerce needs i18n_string and i18n_field. However, i18n 1.8 has a broken i18n_field module.
I've supplied two patches to fix that:
#1904368: [7.x-1.8 regression] Field translation fails in numerous situations
#2015749: Extend support for contrib field types (adds support for commerce_price fields)

Entityreference and taxonomy_term_reference fields don't return localized data from their option list callbacks, which means
that taxonomy attributes on the add to cart field, and search api facets aren't translated.
#2013985: Title replacement doesn't happen for taxonomy term references
#2013849: Term names are not translated
With these two patches the add to cart form can be fully translated.

Commerce patches - (almost) ready

The first two issues contain patches that provide the basic i18n integration, so they are a dependency for all other issues.
#1196488: Allow checkout completion message to be translated
#1121722: Improve DX of instance translation
#2015425: Allow the order creation help text to be translated
#1946302: Allow attribute field titles to be customizable / translatable

Commerce patches - needs work

These patches need to be rebased on the issues above, and completed.
#1270624: Allow tax titles to be translated through i18n