I'm having a problem with accessing subtitles for certain films. It appears to be an issue with those that have accented characters in the file names.
For example, for the short film Getürkt (1996), the mkv is named Getürkt (1996).mkv and the subtitles file is named Getürkt (1996).mkv. When I play the film in Kodi/PleXBMC, there are no subtitles to switch to. If I right-click, however, on the film's menu entry, I am able to select the subtitles through the context menu but they do not appear when playing.
As a test, I renamed the files to remove the umlaut and the subtitles appeared fine and were selectable.
Plex itself has no issue in accessing these films and subtitles as I can get them to display without issue through the browser and through the iOS app, so it looks like either Kodi or PleXBMC is to blame.
I don't have any titles using the ü or Ü characters, but I do have some titles that use Swedish regional characters of similar kind, such as the TV show "Ärliga Blå ögon" and these work fine with Kodi-Helix, using one of the recent PlexBMC versions by hippojay. I just tested it a moment ago, to confirm that these subtitles show properly using this PlexBMC setup. The file names used in this test were "Ärliga Blå Ögon s01e01.avi" and "Ärliga Blå Ögon s01e01.sv.srt". Note that the substring ".sv" in the subtitle filename is optional. It's used only to inform Plex that the subtitle is Swedish, but without that substring the subtitle would still be used, though it would be displayed as "Unknown" language in the GUI, rather than the proper "Svenska". The regional filenames were also used in the show folder name, this being "Ärliga Blå Ögon (1977)". So use of these umlaut characters did not ruin the results for me, like it apparently did for you.
Perhaps your results are somehow caused by some incorrect or missing configuration setting related to regional character use.
For example, In the Kodi submenu 'Settings' > 'Appearance' > 'International', I have the following settings which may have some influence on this:
"Region" is set to "Central Europe" (not quite true, but Northern Europe is not available as a choice, and Central Europe does match the timezone)
"Character Set" is set to "Western Europe (Windows)" (This is the setting I think most likely to have such an influence.)
"'Keyboard Layouts" is set to "Swedish QWERTY"
Best regards: dlanor