Internationalization and Localization
gtkmm applications can easily support multiple languages, including non-European languages such as Chinese and right-to-left languages such as Arabic. An appropriately-written and translated gtkmm application will use the appropriate language at runtime based on the user's environment.
You might not anticipate the need to support additional languages, but you can never rule it out. And it's easier to develop the application properly in the first place rather than retrofitting later.
The process of writing source code that allows for translation is called internationalization, often abbreviated to i18n. The Localization process, sometimes abbreviated as l10n, provides translated text for other languages, based on that source code.
The main activity in the internationalization process is finding strings seen by users and marking them for translation. You do not need to do it all at once - if you set up the necessary project infrastructure correctly then your application will work normally regardless of how many strings you've covered.
String literals should be typed in the source code in English, but surrounded by a macro. The gettext (or intltool) utility can then extract the marked strings for translation, and substitute the translated text at runtime.