There should be an option to turn off spell checking in a note. If there was an icon (for example next to Preview icon) to toggle spell checking on/off it would be awesome. Turning off spell checking should be persistent so if I close note and reopen it the spell checking will be still turned off.
As for disabling spell checking in non English notes there could be something like this: when you write note in Spanish you tag it as lang-es and then in spell check settings there should be an option to turn off spell checking for notes tagged: lang-es, lang-de, etc.
Tagging notes with lang-* could be useful when using external dictionaries, so for notes tagged lang-de use German dictionary and for notes tagged lang-jp use Japanese dictionary and so on.