Please first update the firmware to 3.5.1 or later with which TwonkyMedia Server has been updated to v6.
For .avi/.divx files, TwonkyMedia server checks if a file with the same name but with one of the following extensions (.srt/.sub/.txt/.smi/.ssa/.psb/.ass) exists in the same directory.
File name comparison is case insensitive. Therefore the files "file1.avi" and "FILE1.SRT" are considered matching.
If multiple subtitle/caption files are available for a video item, the server will include only one of the subtitle/caption files in the <res> element. Which subtitle/caption files to be include depends on the order of entries in the file system directory structure, therefore it is undefined – as a user in general only wants to see one language in subtitles, this is not a real issue.