Add multilingual support for generating reports

[Mbb640]

I am trying to generate reports in both English and French, but to do so I have to run tests twice with different language settings and upload twice with the same build hash. The build hash must be mocked for the second run to upload, and since they are separate reports it can be confusing since it appears to be testing two different builds.

It would be great if the duplicate build check also checked the language of the report, and allowed for multiple builds if they have differing languages. The server/viewer itself having i18n support would also be appreciated.

Ideally, the report can be generated in multiple languages at once without needing multiple runs - just swapping the labels without rerunning the tests, but that's a lighthouse issue, not lighthouse-ci.