Currently the staging site only has completely translated languages in the From dropdown. We should expand that to show all languages, including those which have partial translations. This is because the partial translations might be missing translations for labels which don't need them (like numbers - example). Instead of hiding them completely, let's do something like this -

That is, for missing labels, tell the user that the string is untranslated and also tell them the default/English label for the missing translated label.

Preview - The preview image will contain the default/English label in place of the untranslated label. Once the user adds a translation, the preview updates as usual to replace the default/English label with the added translation.