In the standard Ashkenazi Siddur, why is the Aramaic word פורקן used in the otherwise entirely Hebrew prayer of על הנסים?
See the Aruch on the entry for פרק, which cites תהילים קלו:כד :
"And hath delivered us from our adversaries"
Thus, the word is not Aramaic.
This question is a complicated one because Hebrew and Aramaic are sister languages with many shared words and cognates. Even if פורקן was an entirely Aramaic word (which is not at first glance easy to determine based on form or attestation), that does not mean it cannot become a Hebrew word over time. For instance, the word אלא is an Aramaic word (a contraction of אן לא, "if not") that became such a common word in rabbinic texts that we just treat it as a Hebrew word. All languages borrow words (e.g. in English: gesundheit from German, chandelier from French, etc.). Rabbinic Hebrew is full of Aramaisms and straight-up borrowings.