https://docs.mongodb.com/mongoid/master/tutorials/mongoid-documents/#localized-fields says:

> Mongoid now supports localized fields without the need of an additional gem.

This makes it seem that Mongoid has built in localization support, however the support is provided by an external library (i18n) which can make breaking changes in minor releases (https://github.com/mongodb/mongoid/pull/4548, https://github.com/svenfuchs/i18n/pull/415). To work around the breakage users may need to restrict i18n version constraints which is confusing to figure out when Mongoid creates the impression that localization is internal to it.

(Mongoid does not explicitly depend on i18n, this dependency comes out of activesupport.)