This is specific to how you configure Rails to maintain i18n state (unrelated to turbo-frame). If you removed turbo-frame and the links navigated strait to a Rails page, the user would experience the same issue. The Rails documentation outlines multiple options for how to set the I18n.locale, otherwise it rolls back to the i18n.default_locale. Some options for changing the locale from default are:
- params (url, form, etc)
- domain (e.g. example.com vs example.de)
- user config (e.g. stored in DB)
- session or cookie
see: Rails Internationalization (I18n) API > 2.2 Managing the Locale across Requests