Currently, this feature is not fully implemented. We started support for this (hence the export to JSON), but didn't implement it fully. However, as you already noticed the resource strings are identified by $R, so it's pretty easy to update this variable with data coming from a JSON file by yourself.
Once you have the JSON file (for one language) you can alter its values to any other language. Then, when the application has been loaded you have to load the JSON file for the language of choice as well and replace the current values of $R by this.
If not limited in time today, I could write some example code, but it's straight forward. The only problem with this approach is the fact that if you add more variables you have to adapt your JSON files as well. This might be cumbersome in the long run. So hopefully a little support by the IDE will get implemented sooner or later.
What's high on the list is (Eric's suggestions to this topic):
- be able to specify gnutgettext dictionaries at the project level
- be able to populate/update the dictionaries from the resource strings
- during compilation generate json files with localized strings (using the dictionaries)
- maybe be able to mark of form as localizable, it would then use resourcestrings in its auto-generated code rather than simple strings
While these plans have been made, there is currently no roadmap for this part.
Personally, I don't use/need localization so far, thus there's no motivation for me to implement it. But I can only speak for myself here and only for now. It might surely change in the future.