TextTrack LanguageCode parameter in Tracks.BeginCreateOrUpdateAsync

How to set the TrackBase parameter with LanguageCode when uploading track with Microsoft.Azure.Management.Mediav6.0.0?

Adding track works, but languageCode remains empty

await client.Tracks.BeginCreateOrUpdateAsync(
    config.ResourceGroup,
    config.AccountName,
    foundAsset.Name,
    "M0508826_FI.vtt",
    new TextTrack("M0508826_FI.vtt", "suomi", "fi")
);

 {
            "name": "M0508826_FI.vtt",
            "id": "/subscriptions/.../providers/Microsoft.Media/mediaservices/.../assets/.../tracks/M0508826_FI.vtt",
            "type": "Microsoft.Media/mediaservices/assets/tracks",
            "properties": {
                "track": {
                    "@odata.type": "#Microsoft.Media.TextTrack",
                    "fileName": "M0508826_FI.vtt",
                    "displayName": "suomi",
                    "languageCode": "",
                    "playerVisibility": "Visible"
                },
                "provisioningState": "Succeeded"
            }
        }

Tried to set languageCode also to "fi-FI", but result is same. It works fine if the track is added via REST API.

REST HTTP POST body being

{
    "properties": {
        "track": {
            "@odata.type": "#Microsoft.Media.TextTrack",
            "displayName": "suomi",
            "fileName": "M0508826_FI.vtt",
            "languageCode": "fi",
            "playerVisibility": "Visible"
        }
    }
}

Solution

Answering myself, as I found the answer hidden in the comments of the TS sample file at https://github.com/Azure-Samples/media-services-v3-node-tutorials/blob/f614109b34bc5ef8e7a4be074b8e30c99f3fc7c4/Assets/add-WebVTT-tracks.ts#L138.

You need to include the language in the WEBVTT file as follows:

// AMS supports uploading both .vtt files and .ttml files in the IMSC1 text profile (basically no images in base64!)
// The desired language should be set in the head of the vtt file in the following format:
//
// WEBVTT
// Language: en-us