First, I was surprised to see the English text still displayed as spelled incorrect when I opened in my English install of PowerPoint. After that, I figured once the slides originating from the Spanish version of PowerPoint were inserted into an English version PowerPoint, it would correct itself. I found that still wasn't the case. All the slides that were authored in the Spanish version of PowerPoint still retained the Spanish dictionary, and therefore the English text on those slides indicated they were misspelled.
Come to find out, each text box within PowerPoint is assigned its own dictionary language. In order to correct, follow these steps. (Note that I'm using PowerPoint 2007.)
- Go to an individual slide
- Press Ctrl-A to select all object on the slide, including the Title and Body
- Go to Review >> Language
- this will open a Language dialog box
- Change the "Mark selected text as:" option to English (United States) [or whatever language you want to change to]
- Click OK
- Repeat for each slide in the deck that you want to update the dictionary
Unfortunately I couldn't find a way to do this for all slides at once.