Add app localization

[redbassett]

The app will need to be localized both by language (for strings) and country code (for information on which health authority to contact) prior to release. I do not know what the current plan is exactly for official release, so this is likely blocked by knowing where we are releasing various builds of the app.

[crayne]

There is an automated translation service called Smartling (smartling.com) which can be configured so that translation happens automatically on PRs. It is being used by another COVID project, findthemasks.com, and works very well. Maybe we can get Smartling to donate the use of the service.