@WillLi Even in the West, most people think of Satan as a king of demons, rather than a particular fallen angel which led a rebellion against God as per the description from the Christian Bible, so it's not as big a deal for them to use "Satan" as an English localisation for 魔王 since the idea is already there. Of course, it is also used for alliteration purposes, since "Demon King's Secretary" doesn't quite roll off the tongue as well as "Satan's Secretary".
By the way, "Allah" is simply the Arabic word for "the God" and not a proper name either; it's not referring to the Muslim God in particular, so Christians and Jews in the Middle East have also been using "Allah" to refer to God for hundreds of years before Islam was even around, so if something referring to "the God" is localised in the Middle East, they would use "Allah".
Additionally, with regards to 進撃の巨人, I think the main problem is when the author made the title a sort of pun since it refers to both the advancement of the titans and the Titan that Eren had inherited, but this failed to translate well in English.