gettext_i18n_rails_js is currently listed in bundler.d/i18n.rb (and assets.rb) which makes it a runtime dependency, but I don't think it needs to be. It only provides two things: rake tasks (and parsers), and assets which are precompiled.

gettext_i18n_rails_js should be in the assets + development groups only, so it's included when we precompile assets and again for developers who need those rake tasks to build locale files. Ensure that lib/tasks/gettext.rake is updated so it handles the gem being missing when it requires gettext_i18n_rails_js/tasks.

I'd probably suggest rearranging the i18n group a bit and moving fast_gettext, gettext_i18n_rails and i18n_data (see also #8462) to Gemfile as they're required deps, it's not an optional feature - then i18n.rb can be deleted.