Problem (and solution) 1:
In ot-loader.php the method hooks() sets up an add_action() for method load_textdomain(). However hooks() is called last in the class constructor, after admin_includes(), the latter which includes among other files "ot-ui-theme-options.php". That file has i18n'ed strings that as such can obviously never be shown localized as the textdomain isn't yet loaded.
My proposal for a solution would be to call load_textdomain() in the constructor, first thing. I see no reason to delay its call.
Problem (and solution) 2:
OT_LANG_DIR is defined without a leading slash, leading the .mo file to never be found. Prefix '/' in the define and all's well.
One last minor issue: I didn't find it documented anywhere that you need a 'theme-mode' subdirectory in the lang folder for option tree in theme mode. Or did I overlook it?
Thanks for a fantastic plugin!