I would extend this.
Not only spell-checking, but also language setting.
The product should look like:
1. Choose, what language you want your app to be installed in + have the option to change it in the app.
2. Choose what languages you want the spellchecking to be at -> it's normal to use more languages at the same time in one space. The enigne could see, if the language of the note, for example, is one of the selected for the spellcheck, and check for that language.
I wonder, how this can still be a topic nowadays. Big fail, Evernote.