We released our i18n plugin documentation during the StrapiConf 
https://strapi.io/documentation/developer-docs/latest/development/plugins/i18n.html#usage-with-the-graphql-plugin
We had some constraints on relations due to various types (ex: oneToMany, manyToOne, oneToOne, and manyToMany) but we will be expanding i18n support for these in the v4. Originally the i18n plugin was scheduled to be developed on the v4 but we decided to build most of it in the v3 instead however we had to cut certain features due to the framework limitations in the Strapi v3 version.
This is one of the features I mentioned above that was cut for the Strapi v3 version due to framework limitations. We will have more information as we continue the design on the Strapi v4.
Slugs (UIDs) are a unique field type generated either on their own or by referencing a field. Due to the nature of being unique we apply database constraints in the database layer. Since we store all localized content in a single database table you cannot have the same value for multiple entries on this field. UIDs could also be unique globally throughout the entire Strapi application thus in our current v3 framework it is not possible to have multiple localized version that share the same UID. This is something we are looking at changing in the Strapi v4 with our new database layer. More details will come on that in our v4 RFC that should be released later this quarter.