On two occasions the last few months, Simutrans has suddenly forgotten all settings (language, muting of sound) after upgrading. Simutrans is good at loading old paks and old savegames, so why shouldn't it be able to load old settings?
If you used a nightly between r6078 and r6108 the settings were updated to a newer version. r6108 downgraded the version for some reason... that would lose the settings.
Strange (but lucky) that it didn't affect the save games, too.