It officially is 'sleigh beggy' in The Ancient Magus' Bride
In The Ancient Magus' Bride, the protagonist, Chise Hatori, is a special kind of mage, and for a long time there's been some confusion over what English language term should be used for what she is. Specifically, there's been confusion and disagreement over whether she is a 'sleigh beggy' (the official Seven Seas manga version) or a 'slay vega'. Last year I wrote an entry about this, after Crunchyroll translated the term as 'slay vega' in the first episode of the OVA, partly because my personal preference is for 'slay vega'.
Since then, two things have happened. First, Crunchyroll is airing The Ancient Magus' Bride TV series, and in the TV series Chise is a 'sleigh beggy'. More importantly, we now have official confirmation from the mangaka:
@drewtnguyen: @EzoYamazaki00 HELLO! QUESTION! First, thank you for the signature at CRX! Second, how did you come up with the name スレイ・ベガ? Thanks!
@EzoYamazaki00: Hello！Thank you for coming. It is words of the Isle of Man.The words hint at a fairy or a fairy and a human child.
This pretty much settles any debate over the official English term for what Chise is. Crunchyroll has changed its translation (although its subtitles for the first OVA episode still use 'slay vega') and Kore Yamazaki herself has given us an unambiguous answer. The Seven Seas manga translation was faithful to the mangaka's intentions, wonky explanation and all.
(I do wonder how the Crunchyroll 'slay vega' translation for the first OVA episode came about (and if anyone got yelled at over it), but we'll probably never hear that story.)
I still stand by my view that it's an unfortunate choice, but it probably doesn't matter very much, especially for the TV series. Although 'sleigh beggy' does have a meaning, most people are going to read it as a weird arbitrary combination of words, just like they would have read 'slay vega'. Arguably it has less meaning that 'slay vega', because people would probably read implications into the 'slay' part of that.