Crowdin supports all major localization file formats. As far as I know, except for Apple’s new .xcstrings, there are no i18n libraries that use multilingual JSON.
For any file formats not supported by Crowdin, we offer the ability to develop a custom file format and often develop custom file formats for customers. Custom File Format Module | Crowdin Developer Portal
The engineering team would like to know what i18n library or system produced your multilingual JSON? If there is a standard for that file format and we have a chance to reuse that file format in the future, we’ll be happy to develop support for your JSON.
If we can’t, I can recommend that you consider splitting languages into multiple JSON files. One JSON for each target language. Then either load them based on a user’s locale, or compile and merge multiple JSON’s after translation so you don’t have to change your code. You could also consider developing a file format for Crowdin to not bother compiling translated JSON’s into one JSON, but that might be harder to implement.
The best solution here would be making small refactoring in your file so that each language has one JSON file with keys and translations into this language.
If refactoring does not work for you, then you can convert your file into plain JSON for translation and then back to multilingual one for production.
Also, if you convert CSV into JSON, then perhaps you could first upload CSV into your Crowdin project → translate it → and then convert the translated CSV into this multilingual JSON?