What different Spanish dialects are the synonymous names Sandiego and Santiago from?

To get things started, I would say that San Diego is Castillian Spanish and Santiago is Galician.

We have the Mission San Diego de Alcala here in California, which is the first of the mission churches established by Blessed Junipero Serra.
http://www.missionsandiego.com/
From there we can go to Alcala, Spain (following my train of thought) which is right in the middle of the region wherein Castillian Spanish is spoken.
https://en.wikipedia.org/wiki/Alcal%C3%A1_de_Henares
https://en.wikipedia.org/wiki/Languages_of_Spain

Then we have Santiago de Compostela in the Galician region of Spain, that some have reached by honest walking, while others took the tour bus : )
"Santiago de Compostela (also Saint James of Compostela) is the capital of the autonomous community of Galicia and a UNESCO World Heritage Site. Located in the northwest of Spain in the Province of A Coruña...."
https://en.wikipedia.org/wiki/Santiago_de_Compostela
https://en.wikipedia.org/wiki/Languages_of_Spain

What is fairly sure is that both derive from the Hebrew for Jacob and that Santiago refers to Saint James while San Diego can refer to Abraham's grandson Jacob or also to Saint James.
"The short answer is that languages change over time, and the original name of Ya'akov in Hebrew changed in different directions in Spanish and English. In fact, both Spanish and English have several variations of that old Hebrew name, of which James and Diego are the most common, so technically there are several ways you could translate those names from one language to another."
"The etymological change in Spanish is not as well understood, and authorities differ on the details. What appears likely, although not certain, was that the Iacomus became shortened to Iaco and then Iago. Some authorities say that Iago became lengthened to Tiago and then Diego. Others say the phrase Sant Iaco (sant is an old form of "saint") turned into Santiago, which was then improperly divided by some speakers into San Tiago, leaving the name of Tiago, which morphed into Diego."
http://spanish.about.com/od/historyofspanish/a/diego.htm

After all that, I am not absolutely certain that San Diego is Castillian and that Santiago is Galician.

I know nothing of Spanish dialects, but I do know the Book of James in the Spanish Bible is titled "Santiago," which makes me thinks that's the Castilian version. I can't argue against the fact that Santiago de Compostela is in Galicia, though.

Go to this site and scroll through the Romance language renderings of the title of James. No Diegos! Just Santiago, Tiago, Giacomo, Jacques, and Iacob. (Check out the Germanic ones for fun.)

In Romanian, though the title is "Iacob," the first word of chap. 1 vs. 1 is "Iacov." Likewise, in the 1569 Spanish (espanol) "Reina-Valera Antigua" version, the title is "Santiago" while the author calls himself "Jacobo."

http://www.biblegateway.com/passage/?search=James%201;&version=6;

This is a question I've often wondered about myself, and I hope someone knows or can find the answer. Another Spanish variation, and a much more apparent cognate, is "Jaime," also appearing as the surname "Jaimez."

You can also take a peek at some variations of James here:
https://en.wikipedia.org/wiki/James_I_of_Aragon

and here is James in Galician:
http://gl.wikipedia.org/wiki/Xaime_I_de_Arag%C3%B3n

(Good old Wikipedia has that clickable list of languages to the left.)

Here's this on "Diego" for good measure, with a 24-name map at the bottom.
http://www.babynamespedia.com/meaning/Diego

Still not finding anything more specific than "Iberian."