On crowdin server $formatRelative (and other APIs) should use the interpolated locale (and not 'ach-ug')

[jonboiser]

Summary

1. Adds a guard in the i18n setup code that pulls the real language code from localStorage (on the Crowdin server) and uses that to configure VueIntl's bootstrapping.

References

Fixes #7330 , but this is only confirmed locally. Will need to reconfirm once it is redeployed to the translation server.

Reviewer guidance

@pcenov @radinamatic

Per #7330 (comment), local testing might not be perfect because of some issues with the state of the localizations. Regardless, the fix for the "relative time" message on the Facility > Data page in #7330. Can be tested by taking the code or generated pex and running kolibri start with the translation settings:

kolibri start --settings=kolibri.deployment.default.settings.translation

For some reason, not all of the text will be localized (especially the "Generated ..." message in the screenshots), but, in my testing, the relative time localization on the Facility > Data page does look to be fixed for all of the languages supported by Kolibri.

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[rtibbles]

Was going to make a further suggestion about setting the languageDirection too - but then realized that would require more extensive changes, as we would need to appropriately load the directional CSS too, and that is bootstrapped into the page.