A nice bit of background on “James”, from Johnson:
THIS is à propos of nothing, but I’ve always wondered why English-speakers call a certain Christian saint “James”, while he is called “Santiago” in Spanish. That led to some scraping around on Wikipedia, which always reminds me that linguists must love editing there; the articles are usually shockingly good. Anyway, the story is quite a whirl. Ready?
The original name is Hebrew’s Ya’aqov, the same as the Old Testament patriarch known in English as Jacob. Jesus’s brother “James” had that same name, which is given as Iakobos in the Greek of the Gospels. From these two, you can see where Russian and other Slavic languages get Yakov, German gets Jacobus and so forth. The French Jacques is no longer a stretch. And Santiago? Well, it's just Sant Iago, Saint Iago(v).
James, then, is still an outlier. Where did that "m" come from? Apparently the late Latin Iacobos turned into a Iacomos. First, the “o” became nasalised (like the French vowel in bon), which made that “b” become “m-like”. Iacobos -> Iacombus. Then the “b” was “simplified” out of existence, leaving Iacomus, and on to James. (How the J that most Europeans pronounce as a "y" sound became our "dzh" sound is a story for another day.) Cousins with an "m" are the Irish Seamus and the Scottish Hamish.
Johnson: answering questions you never asked.