I am not familiar with webvtt, but would the following work for you?
"key": "Videos",
"title": "Videos",
"fields": [{
"key": "video_file",
"label": "Video File",
"name": "video_file",
"type": "file",
"required": 1,
"return_format": "array",
"library": "all",
},{
"key": "subtitles",
"label": "Subtitles",
"name": "subtitles",
"type": "repeater",
"required": 0,
"min": 0,
"max": 0,
"sub_fields": [{
"key": "language",
"label": "Language",
"name": "language",
"type": "select",
"required": 1,
"choices": {
"English": "English",
"Dutch": "Dutch"
},
"return_format": "value",
},{
"key": "language_file",
"label": "File",
"name": "file",
"type": "file",
"required": 1,
"return_format": "array",
"library": "all",
}]
}]