This frequency of English localization issues is unacceptable

[Ransu]

I want to clarify something. There is huge a difference between Old English and ye olde stuff.

FFXIV dialogue relies heavily on archaisms and obsolete features; but even characters like Urianger are speaking Modern English (albeit with uncommon syntax, some Early Modern words, and "thee, thou" sprinkled in for effect). I think there is a very fair comparison to be made with Shakespeare, but nothing before his time.

Shakespeare's first poem was published in 1593, in Early Modern English (~1500-1750).
It's tempting to consider his works Middle English because it was long ago, but they were not.

Old English (~450-1150) would be like Beowulf and The Exeter Book, or poets like Cædmon, Cynewulf, etc.
It looks so different, it isn't mutually intelligible with the English that we read. The vocabulary/roots were purely Germanic until the Norman Conquest of 1066, which begins the influence from Romance languages. The upper class of England started speaking Anglo-Norman French and OE began developing into Middle English over the next few centuries. OE grammatical features were greatly simplified during this time period.

Compare Shakespeare's Sonnet 130 to The Seafarer, a poem written in OE between 450-940. (or this)

TLDR: If the dialogue was in any form of Old English, we wouldn't understand the story

You're taking things a little too literal here. Obviously actual old english is not the same.