I got JSON files structured like this:

{
"fields" : {
"body" : {
"en-US" : "Captures the voice of the user in front of the device and suppresses other voices. \nUse this setting when you want your computer to pick up only your voice instead of voices behind or around you.",
"character_limit" : 50000
},
"title" : {
"en-US" : "Single voice mode",
"character_limit" : 255
}
}
}

I can specify the fields that contain Length Restrictions, but how to make them display as non-translatable?

With these settings, the length restrictions are just excluded from translation, like with default JSON processing.