It basically traces itself back to early Christian usage. Christians still called Saturday the ‘sabbath’ but Sunday is often called “the Lord’s (day),” kyriakē hēmera in Greek and dies dominica in Latin. It’s apparently a very early term - notice how in Revelation 1:10 John says that “I was in the spirit on the Lord’s Day.”
The word for Sabbath is a variation on the number seven in Hebrew. The Sabbath is the seventh day when God rested from creating the world. Many languages use a variation on Sabbath but it is correct because Saturday is the 7th day.
They’re not really ‘slangs’ - in a linguistic sense, they’re each separate languages which have a common origin (Latin). Castilian (i.e. Spanish) is of course one language, Galician (which is very close to Portuguese) is another, and Catalan (related to the Occitan languages spoken in southern France) is yet another. In addition, there’s also other local languages like Aragonese, Mirandese, or Extremaduran.
This is false. Just read about Saturday and Sunday above… And you probably meant all of the names of the week for Spanish… I highly doubt Jose has an occult meaning… along with other millions of Spanish names…
I grew up in the Philippines, and I can tell you that 400 years of being a Spanish colony did leave its mark on the local language(s). (Though we do have a weird knack for spelling de la as dela. :D) For the record, I personally don’t like French: they have a weird pronunciation of u. Portuguese is more interesting, if only they could do something about writing stuff like São João and reading it very nasally like so(w) zhu-wo(w). At least Galician is easier on the eyes because it’s almost written like Spanish.