Add translation support for extension strings

[cwillisf]

The support code in extension workers should provide a way to translate strings.

See proposal: https://github.com/LLK/scratch-vm/wiki/Extension-translation-straw-cat

[chrisgarrity]

@cwillisf isn't this implemented (at least for pen, music etc.)

[cor3ntin]

Any news on that ?

[chrisgarrity]

@cor3ntin translation is already supported for Scratch Team extensions. All the current extensions in the library are localized. This is backlogged so we can make sure that translation is also possible when we support 3rd party extensions. That's not planned until 2019.