In addition to answer provided by Mastergalen and to address comments regarding newly type text:

If you will notice, that language will automatically change back whenever you start to type new text (which is very annoying), you have to change current default language for PowerPoint:

• make sure PowerPoint window is an active window
• in the Windows Taskbar (yes, actually not in PowerPoint), check if Language bar is visible,
  • if not go to Control Panel > Region and Language > Keyboards and Languages. Click Change keybords..., switch to Language bar tab and check Docked in the taskbar option. (this is from Win7, so might be a bit different in other versions).
• now key action - in the Language bar in the taskbar, click language code and switch to EN (if you want currently to use English in PowerPoint). From now on, all new text in PowerPoint will be in the selected language :-)
• if you want write in your original language, just change it back.

In addition to answer provided by Mastergalen and to address comments regarding newly type text:

If you will notice, that language will automatically change back whenever you start to type new text (which is very annoying), you have to change current default language for PowerPoint:

• make sure PowerPoint window is an active window
• in the Windows Taskbar (yes, actually not in PowerPoint), check if Language bar is visible,
  • if not go to Control Panel > Region and Language > Keyboards and Languages. Click Change keybords..., switch to Language bar tab and check Docked in the taskbar option. (this is from Win7, so might be a bit different in other versions).
• now key action - in the Language bar in the taskbar, click language code and switch to EN (if you want currently to use English in PowerPoint). From now on, all new text in PowerPoint will be in the selected language :-)
• if you want write in your original language, just change it back.