re-add translations support

[andrewharvey]

osmlab/editor-layer-index supports translations of source names, however this was dropped during the fork just because it needs to be setup in Transifex. We should re-add this support again once we have a project setup on Transifex.

[comradekingu]

Happy to help if you move away from Transifex and use Weblate instead.

[andrewharvey]

I have no opinion on this, Transifex has the advantage that it's already supported by editor-layer-index, but if you want to maintain an alternative that's fine with me.

At the moment source name translations are still able to be setup via yaml files directly in this repository.

We don't have any sources which really need translating so far, so maybe address this once we include a source that needs translation?