From what I see, it seems that Notepad either detects the language automatically, disregarding system settings, or just checks against whatever number of dictionaries it can call upon. "This is a prueba" not being detected as wrong makes me lean towards Notepad just running words against all available dictionaries, and as long as the word is in one of them, it won't highlight it.
English and Spanish for this test:
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On the other hand, it doesn't highlight either metre or meter, so I guess the word "realise/realize" was a one off error?
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