wxWidgets - wxWidgets: Cross-Platform GUI Library
Crowdin - Localize your product in a seamless way
Electron - Build cross platform desktop apps with web technologies
POEditor - The translation and localization management platform that's easy to use *and* affordable!
GTK - GTK+ is a multi-platform toolkit for creating graphical user interfaces.
Lokalise - Localization tool for software developers. Web-based collaborative multi-platform editor, API/CLI, numerous plugins, iOS and Android SDK.