“Allah” is the Arabic word for God, and Arabic is the language of many important Jewish commentators, not the least of whom is the Rambam. Kaddish is “praise of Allah”. Arabic was also the language of the Jewish population of Eretz Yisrael in the 51st to 56th centuries. “Allah” is not a “Muslim” word, as Muslims speak many languages (including Farsi, Urdu, Hindi, the gazillion languages of Indonesia, and, c”v, English, French and German).
Neturei Karta is meshugenah, but their praise of Allah is merely a use of one of the many languages spoken by Jews, and praise of Allah is a mitzvah.