Agreed. It’s not automatic, and there are requirements to be met. The following quotes are from the law.
“Possibility to request the acquisition of Portuguese nationality, by naturalization, fulfilling the other requirements required by the Nationality Law (Law no. 37/81, of 3 October, with the current wording).”
Two of those requirements are:
"b) Residing legally in Portuguese territory for at least five years;
c) Know the Portuguese language sufficiently;"
Version 11 - the most recent version of the law (Organic Law No. 2/2020, of 10/11)
See Article 6 - Acquisition of nationality by naturalization
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