Is the Japanese well-translated to "kalos" here?
The two lines you quote are: 


  YUI: あのう、アポロンさん、大丈夫ですか。
  
  APOLLON: え？ああ、僕なら大丈夫。カロスだよ。


The word I have emboldened above is カロス karosu, which is just Greek "kalos" written with the Japanese syllabary.

Of course, just as no English speaker (save perhaps a Classicist) will know what the word "kalos" means, no Japanese speaker will know what it means either. This being the case, retaining the word "kalos" when rendering the line into English seems like an acceptable choice; it's equally incomprehensible in either language.

If the show never explains what Apollon means by "kalos", I suppose that the Japanese audience is also supposed to infer what he's on about from context.