Has he used it for other people than the Strawhats actually?
It seems to me that he only started to use it because he said it once while speaking to Luffy and just continued with the rest of the crew, but maybe I forgot a few instances...
Yes, I think he used it for other people too... Maybe I'm wrong
Well I'm pretty positive he doesn't call Bepo "Bepo-ya".
So unless there's a quote or anything I'm pretty sure it was only Luffy and people related to him X:. I even remember Buggy telling Law that he thought it was wierd he called him "Mugiwara-ya" xD.
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 It's just an old outdated way of referring to people.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.