[I18N] Add i18n support for plan features description

The data fetched from the API https://customers.gitlab.com/gitlab_plans is hard-coded.

So no matter what language the user chooses, only the introduction in Chinese can be displayed.

Upstream also has the same problem, but they can only display in English.

Screenshots

(The user currently selects English, but the introduction in Chinese is displayed.)

cc EM @prajnamas