I dug up some cool history trying to answer this.
The person who worked on localisation for Xenogears is Richard Honeywood who was brought in to manage localisation at Square Soft after Final Fantasy 7 received a very poor localisation effort.
"Honeywood described Xenogears, his first translation project at Square and the first to be handled internally by the company, as "pure hell". This difficult experience catalyzed many of the changes to the company's approach to localization, moving booths to always work very closely with the original development teams, improving communication with them, and introducing full-time editors. Another key change was adding a familiarization and glossary creation period to the schedule, in which the team develops style and characterization guides for the project. For Honeywood, a good localization takes into account the cultural differences between Japan and western territories. This sometimes involves rewriting dialogue or altering graphics, animations, and sounds."
He was also responsible for setting the high bar of localisation Dragon Quest is still known for today.
"After Square merged with Enix to become Square Enix, he was tasked with managing localization for the Dragon Quest series. In order to differentiate the series from Final Fantasy, Honeywood decided to localize Dragon Quest VIII: Journey of the Cursed King and future titles in the series in British English. As part of this work, he wrote a comprehensive style guide to standardize names across the entire series, which has been maintained and updated by other teams since."
In terms of 8-4 according to wikipedia...
"The team cites Richard Honeywood, founder of Square's localization department, as an influence on their translation style. Beyond merely translating the words, 8-4 attempts to convey the same experience as that of the original language version through attention to tone, user interface, and cultural references."
8-4 are on good terms with Honeywood so I imagine there was some cross pollination between Xenogears and Xenoblade Chronicles X. He appeared on the 8-4 podcast in 2011 to talk about his work, I'd love to listen but for fear of spoilers I'm going to wait until I've finished the game.
http://8-4.jp/blog/?p=381After leaving Square Enix in 2007 he went to Blizzard and then in 2010 to work at Level 5 on Ni No Kuni, now I presume he's working on Ni No Kuni 2.
8-4 also worked on Baiten Kaitos Origins and Xenosaga 3 meaning they were probably well acquainted with Monolith Soft making them good candidates to handle the majority of Xenoblade Chronicles X. One more fact I seem remember is that someone on the 8-4 Play podcast said that localisation began around October 2015, and that the voice work was done around June/July/August 2016.
https://en.wikipedia.org/wiki/8-4https://en.wikipedia.org/wiki/Richard_Honeywood