In The Ancient Magus' Bride, the protagonist, Chise Hatori, is a special kind of human magus. As I wrote about in passing once before, the English language term that should be used for Chise's status is somewhat unclear; the initial fan translations of the manga used one term and then the official Seven Seas version used another. Then something interesting happened when the first OVA episode came out recently on Crunchyroll, as I wound up discussing on Twitter.

@cks_anime: Well I'll be. The Crunchyroll subs for Ancient Magus' Bride do indeed call Chise a 'slay vega', not a 'sleigh beggy'. That's … interesting.
And the prob[able] reason that CR uses 'slay vega' for Chise in Ancient Magus' Bride is that really seems to be what the character actually says.

This happens at about 14:03 in OVA episode 1, if you want to see and hear it for yourself on Crunchyroll.

(When I initially watched the OVA I didn't really notice this usage, but I was recently thinking back and suddenly it stood out, so I checked back to make sure that I wasn't misremembering because of my biases.)

I think this means the Japanese dialog directions went out of their way to use 'slay vega'. So why did the US manga make it 'sleigh beggy'? ♯

I may be predisposed to hear an ambiguous couple of words this way, but it certainly seems that the fairy's seiyu is specifically saying 'slay vega', not something ambiguous. This is especially interesting because my understanding is that Japanese does not normally have 'sl[a]' or 've' phonemes; if this is correct, the seiyu has to be carefully going out of their way to pronounce this 'right', and the Japanese script likely had to call for that.

@halfadeckshort: I was curious. It looks like they [Seven Seas] answered a couple of questions on tumblr. (link) (link)

@cks_anime: Yep, their pre-OVA claim is that 'slay vega' was a fan mistranslation. But if the JP OVA dialog uses 'slay vega', well...
So, did the original mangaka say 'not slay vega' to Seven Seas & later 'sure, slay vega' to the OVA writer?
Or did Seven Seas decide to use 'sleigh beggy' without asking the Japanese side about it, then get ambushed by the OVA?

It's not clear to me who wrote the OVA. Wikipedia co-credits Kore Yamazaki, the mangaka, and Aya Takaha; MAL lists Aya Takaha as 'script' and Kore Yamazaki as 'original creator'; ANN has Kore Yamazaki as 'scenario' and Aya Takaha as 'script'. On the whole it seems most likely that Aya Takaha wrote the dialog but that Kore Yamazaki was clearly involved and thus available to be consulted for things like 'how is this term pronounced'.

@cks_anime: I suspect that what happened is that SS didn't want to use the fan translation, didn't ask the JP side & just picked a name.
What term Seven Seas use in the next translated volume will be quite interesting, since it'll be the first post-OVA volume.

Looking back, Seven Seas' first answer does not explicitly say that they asked the Japanese mangaka or publisher what the English language term should be, although it certainly sort of implies that they did. The 'translator, adapter, editors, etc' could have been entirely the team within Seven Seas.

My personal preference is for 'slay vega', so I am happy to have it validated by Crunchyroll and apparently the Japanese OVA and I hope that Seven Seas switches the term with the next translated manga volume (currently said to be released in January). Also, of course, right now 'slay vega' seems more likely to be what the mangaka actually intended, not 'sleigh beggy'.