I've created a localized Wix Bundle installer for 28 different languages. I've tested it on my two different machines and it works fine, showing the UI in English on the English Windows machine, and the UI in Spanish on the Spanish Windows machine.
How can I force the Wix installer to run in a specific language so I can test the installer is ok for all the other 26 languages without having to have Windows versions for each languages?
That depends on the bootstrapper application you used. If you use WixStdBA, you can use its /lang <lcid> switch.