Yes, I think he used it for other people too... Maybe I'm wrong
He calls Eusstass-ya too. I guess it is related to who are his friends as he grows up. According to some theories, Trafargar Law is a representation of a pirate from England. And we all know England for some people uses more polite words when talking, don't you think, sir?
I wouldn't know, my native language isn't English and I'm not European either. I know about the stereotypes but I'm unsure what's exagerated.
But ya was explained by Oda in a SBS from volume 62.
"O: Mr. Law—they're calling you --! ............. Aw he won't come-. Let me answer instead. A long time ago, around maybe the Edo period--. There was this thing called "Yagō". So if for instance, in the case where there were two Mr. commoners named "Tomekichi-san", things like "Dōgu-ya no Tomekichi" (Tomekichi of the Tool shop) or "Oke-ya no Tomekichi" (Tomekichi of the Bathtub maker); something with "~ya" would be used in place of a last name-. Like how you shout "Tamaya~" at the fireworks or "Nakamuraya" from Kabuki; have you heard of those? In other words, he just goes with the flow. Right, Law-san?"
Doesn't seem like he's trying to make him sound English xD.
By the way, Law responded with "L: Yeah.".
Ah, well maybe he uses it for people he thinks as rivals. Supernovas and such. Personally I don't remember where he referred to Eustass that way; what chapter?
But "-ya" isn't perticularly polite or unpolite. It's just an old outdated way of referring to people.
I was thinking... When Law was a child, he didin't call people with the tag "-Ya", he didin't spell Doflamingo-ya or Baby 5-ya, right? I wonder when or how did he got that habit. Because he calls Mugiwara-ya, Nami-ya... or anybody that way, I used to think it was an accent (but it's not) lol.