Our nonlocalized web part manifest (FeedbackWebPart.manifest.json) looks like this:
The language dependent parts have default properties. If you want to translate those parts to other languages, you do it by adding new language-specific properties and values.
There is one problem in the web part manifest. You can't localize the default values for the properties as easily as other values. There are two ways to do it: either you create multiple preconfiguredEntries objects to support multiple languages or you handle default property values in your code using the strings object as you do with the other language dependent text. The problem with multiple preconfiguredEntries objects is that you usually only want to show the web part to the user once, when he or she is adding it to the page.