I've bounced between a few fan projects over the years, and my loyalty ends up with whoever is currently translating the main story chapters. The Atlas Academy database is an indispensable tool for pure gameplay text—skills, Noble Phantasm descriptions, and event details are impeccably sourced from the game files themselves. For the narrative, the 'FGO Materials' translations by community groups on Reddit or Beast's Lair tend to be the most reliable for lore-heavy stuff, though you have to hunt a bit as they're scattered. The official English release is, of course, the baseline, but its pacing and sometimes localization choices for cultural references can feel a step removed from the original's nuance. So I cross-reference: official for the main read, then dive into those community-translated material books and thread discussions for the deeper cuts. It’s a patchwork method, but it gets you the clearest picture.
Honestly, the 'most accurate' translation is often the one that explains the context, not just the words. I've seen fan translations of 'Fate/Grand Order' that include lengthy translator's notes about a specific pun or historical allusion, which feels more truthful to the intent than a smoothed-over official line. That kind of effort usually lives in dedicated Discord servers or old-school forums, not on the big aggregate sites.