Id say those are subtitles. Either in the same folder (srt file) or they are included in the mkv. You can use a tool like mkvtool to remove them from the file or just turn them off in whatever player you are using.
You are right, it's part of the subtitles. Not sure why I did not think of that.
Also thanks for recommending mkvtool, never heard of it until now which is great because when I play LOTR from a flash drive on my TV, it defaults to Italian language.