For me the problem wasnt the language settings since I was already at English. I had clicked on the "Disable new Acrobat" option in the Menu tab. After I reverted back to the new Acrobat by clicking on Menu-"Enable new Acrobat", then I was able to locate the AI Assistant in the "See all tools" link. Afterwards, and not related to my inital problem of not finding the AI Assistant, I aslo checked "Enable Generative AI feature in Acrobat" in Menu-Preferences.
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