fix issue where regional locales are not matched against our locales list

[adonesky1]

Fixes: #12353

Explanation: Some users experience issues in onboarding if their browsers' language settings are set such that their primary language is coded regionally, and another secondary language - for which we have translations - is not. Before this PR we were not normalizing regionally coded language settings to their base (e.g. 'en-us' => 'en') if we didn't have that regional coding in our translations. This PR adds this normalization.

Manual testing steps:

• Change the language primary language in your browser's preferences to one with regional coding (e.g. French (Switzerland), code 'fr-CH'), for which we have translations for the non-regionally coded (in this case just French (code 'fr')
• Reload the app
• Check that the language is set to the language you just changed it to (French if you followed step 1)

[github-actions]

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[metamaskbot]

Builds ready [baada80]

• builds: chrome, firefox, opera
• build viz: Build System
• code coverage: Report
• storybook: Storybook
• all artifacts
• bundle viz:

  • background: 0, 1, 2, 3, 4, 5, 6, 7, 8
  • common: 0, 1, 2, 3
  • content-script: 0
  • contentscript: contentscript
  • disable-console: disable-console
  • inpage: inpage
  • phishing-detect: phishing-detect
  • policy-load: policy-load
  • sentry-install: sentry-install
  • ui: 0, 1, 2, 3, 4, 5

Page Load Metrics (397 ± 42 ms)

Platform	Page	Metric	Min (ms)	Max (ms)	Average (ms)	StandardDeviation (ms)	MarginOfError (ms)
Chrome	Home	firstPaint	281	532	396	85	41
		domContentLoaded	272	521	384	85	41
		load	281	529	397	87	42
		domInteractive	272	521	384	85	41

[Gudahtt]

LGTM!

[metamaskbot]

Builds ready [7102ee9]

• builds: chrome, firefox, opera
• build viz: Build System
• code coverage: Report
• storybook: Storybook
• all artifacts
• bundle viz:

  • background: 0, 1, 2, 3, 4, 5, 6, 7, 8
  • common: 0, 1, 2, 3
  • content-script: 0
  • contentscript: contentscript
  • disable-console: disable-console
  • inpage: inpage
  • phishing-detect: phishing-detect
  • policy-load: policy-load
  • sentry-install: sentry-install
  • ui: 0, 1, 2, 3, 4, 5

Page Load Metrics (390 ± 42 ms)

Platform	Page	Metric	Min (ms)	Max (ms)	Average (ms)	StandardDeviation (ms)	MarginOfError (ms)
Chrome	Home	firstPaint	290	603	389	87	42
		domContentLoaded	280	601	378	88	42
		load	289	601	390	88	42
		domInteractive	280	601	378	88	42