Translation quality

[Saccharin]

I heard the english translation added lots of extra fluff and actually even altered some personnalities, how true is that statement ?

You should think of it more as a localisation than a translation.

Something's like jokes for instance that might be funny in Japanese don't translate well into other languages. Take the endings of spells for ranks in Japanese, they work logically in Japanese text but don't in English so SE (NA) game them numbers, to some controversy. If you also look at some fate names in the English client you can see references from English pop culture.