One big surprise, when working with multiple languages, is that loanwords often have different meanings from the original language. This is natural, of course, as languages change and evolve over time, but it’s always a bit of a surprise to run across.
For example, English has a few loanwords from Japanese that are now almost unrecognizable: “tycoon” in English refers to a rich and influential businessperson, but it comes from 大君 (taikun), an old term used to refer to a sovereign (akin to “his majesty”). Likewise, the word “honcho” — usually “head honcho” — is used in casual American English to mean “the person in charge,” brought back after the soldiers in the postwar occupation learned the word 班長 (hanchō, “group leader”).
Of course, many English words have different meanings in Japanese. オリジナル (“original”) in Japanese is more or less exclusively used in the sense of “originality,” or “new and unique,” as opposed to its usual English meaning of “the first.” On the other hand, グッズ (“goods”) is closer in nuance to “merchandise” in Japanese, generally being used to refer to, say, licensed character merchandise (“character goods” in Japanese) or sporting goods (スポーツグッズ). Despite being a perfectly fine phrase in Japanese, referring to “original goods” in English is going to raise some eyebrows — something like “commemorative items” or some form of “merchandise” would be much closer in meaning to the Japanese, depending on the context.
These are good examples of how many loanwords change: often, a loanword has a narrower meaning than the word in the original language.* “Sauce” in English can mean any type of sauce, such as dipping sauces or béchamel sauce or barbecue sauce, but the loanword ソース in Japanese by itself usually refers something more along the lines of Worcestershire sauce specifically. On the other hand, English does the same thing: the word “salsa” was taken from Spanish, where it simply means “sauce.” As a result, the Japanese word サルサソース (“salsa sauce”) effectively means “sauce sauce”!
*Once in a while, interestingly, the opposite happens. For example, the English word “juice” has been adopted by Japanese as a loanword and, over time, has come to also encompass basically all other cold, sweet, non-alcoholic drinks, such as, say, cola.