aka i18n (the 18 stands for the 18 letters between the i and final n in the word 'internationalization').
For details, see i18n - Writing for the world, i18n tester, Encoding Translations and i18n.
See also http://www.tcl.tk/doc/howto/i18n.html - How to Use Tcl 8.x Internationalization Features.
A developers view on Internationalization and the Tcl C APIs
Note that programs that have undergone internationalization should also go localization before being shipped to customers.