Illustrate translation coverage in language selection dropdown

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In order to set expectations when picking a new language from preferences, we want to show language coverage in the dropdown.

Proposal

• In the drop-down, show to the user the % of language translated.
  • #328430 (closed)
  • #329852 (closed)
  • #329326
• The dropdown should be styled as designed.
• The actual percentages should be updated automatically (vs. the current manual process).

Considerations

Given that not all strings are externalised, saying a language has say 90% coverage, isn't currently a true reflection of how much of the user interface is in fact translated

Design

• Try to update settings dropdown ui to use new UI, see https://gitlab.com/gitlab-org/gitlab-ce/issues/22223