@leocg I have 1 other computer with Opera Portable on it and that one has a spellchecker for Spanish (Venezuela). I don't really understand why the regular installed version of Opera wouldn't also include it, hence my confusion and why I hadn't already tried adding a different Spanish.
Either way, adding the generic Spanish language seems to have done the trick in activating some form of Spanish spellchecker and that should be fine for my needs.
Thank you!