Use selected language for greeting date and calendar (#663)

[acthelemann]

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Please include the issue number the pull request fixes by replacing YOUR-ISSUE-HERE in the text below.

Fixes #663

Summary

• Greeting date uses selected language
• Calendar uses selected language
• Fix bug where going to settings page would reset language to English

Test Plan (required)

Final Checklist

• [x] For CoronaTracker, did you bump the version in package.json?
  • Update the second decimal for a major change
  • Update the third decimal for a minor change
  • Numbers can go past 9, e.g. 1.0.9 => 1.0.10
  • For more info, read about Semantic Versioning
• [x] Did you add any new tests as necessary?
• [x] Is your PR rebased off the most current master?
• [x] Have you squashed all commits? (can be done at merge)
• [x] Did you use yarn not npm? (important!)
• [x] Did you use Material-UI wherever possible?
• [x] Did you run yarn lint on the code?
• [x] Did you run all of your most recent changes locally to make sure everything is working?

[AdhamAH]

Oh! I should take a look at this before merging the other PR. You need to rebase and update JSON. It would have been easier if I didn't merge that one😅. My intention to give less work not more.