Maybe I've missed it, but I looked through the manual, wiki, and issues here, but couldn't find any more details about the spell checking mechanism.

The spell checking, which is done by Ipe in text fields, seems to depend on the currently set locale. On my Linux system (which is set to English) I can thus force German spell checking (for German slides) by launching Ipe as LANG=de_DE.UTF-8 ipe. However, it would be nice to have an option to set the document language in the file options and having the spell checking respect the setting.
I think a good place, if considered, would be the document properties.

If one wanted to go further, it might even be possible to set a document wide language in the document properties and still have the option to locally change the language for specific text boxes.

However, I guess that this really is not an urgent problem, as I can easily work around it. I just thought I might leave these thoughts here, even for other people who have wondered about changing the spell checker language.