Add language mapping to crowdin.yml

This adds all our languages to crowdin.yml.

Since we currently use 2 ways of identifying our locales, after discussing with the Crowdin support team, I think it makes sense to define all languages to map to the default.

In a later stage, we could start using this file ourselves to avoid duplication in Gitlab::I18n.

Required for the crowdin integration in https://gitlab.com/gitlab-org/gitlab-ce/issues/33521