[WFX-387] [WFX-388] Add translations for several languages.

[adamp01]

For reference: including extensibles.ftl directly into browser.jar broke things as soon as localised extensibles.ftl were present. Attempting to put extensibles.ftl into en-US.jar to allow us to have extensibles.ftl in a custom directory caused even more issues, so have resorted to placing the en version in browser/locales/en-US/browser.