Language won’t fully change
I switched my phone to another language, later I said it back to English but sometimes when I would search a word through the iPhone search bar it would pop up in the second language.
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Make sure the other language is still loaded onto the phone. It needs to be beneath English (or your desired one) in the Language and Region settings for it not to change the primary system language.
You could also add the dictionary for that language and for proper spelling and grammar correction add another keyboard.
All those things are found in the same section of Settings.