In ubuntu-system-settings, selecting a different language shows a popup saying that the phone needs to restart. If you select cancel at this screen, it appears to have changed the language anyway. However, if you go back into the language selector and try to choose the language you came from, it won't let you.
Steps to reproduce:
1. open system-settings->Language & Text
2. select "display language" at the top
3. choose another language (ex. Español)
4. The buttons at the bottom change to Cancelar and Confirmar, select Confirmar
5. on the screen that comes up, the two options are "Reiniciar" and "Cancelar" - select "Cancelar"
- This now takes you back to the langauage and text screen, but now it's "Idioma y texto"
6. Select the top option to select a langage again, notice that it will not let you confirm the selection of the previous language
One extra note... it won't let you choose the original language setting, but if you cancel that UI the rest of the phone will revert back to that original language anyway.