Bob is not that old but he can process information a lot faster than Harry so who knows.
Bob is just plain
smarter than Harry! Though blindingly naive & ignorant in some ways.
IIRC Bob used to be human, in the Renaissance-ish era...? I doubt he spoke Old Welsh in his lifetime, but I suppose it's possible.
But since death, he has been a dedicated magic-learning spirit.Edit: per WoJ, Bob never was human. But that doesn't actually alter the following point:
I (strongly!) suspect that
the original Merlin's native tongue is something
a dedicated magic-learning spirit knows! I don't doubt the journals are among the most significant books Merlin left behind, but I strongly suspect there
are others... plus other relevant books written in Old Welsh
just because it was the Merlin's tongue.
All fast methods imply sharing the information with someone and Harry might not want to do so. He can try to learn the language but he is not a fast learner of language if his experience with latin is indicative. It will take real effort over years.
There might be tricks to speed it up if Harry is prepared to leave his body for a while and stretch subjective time the way Bob does but...
Lash spoke a bunch of languages.
She was information in Harry's brain. So the ability to translate multiple obsolete languages can be conferred very very quickly! And Harry's notoriously-bad ability to learn Latin can be bypassed.
Now... that may be a trick only known to Angelic-origin beings. Even Mab may not know it, and I'm
certain that "ordinary" fae don't! Similarly for Vadderung. Or one or both of them may know the trick; we just don't know.
OTOH, we know Jim plans to introduce other pantheons to us. Maybe Cerridwen (Welsh goddess of knowledge & magic!) might be a source, or Hades (who likes Harry) might simply be one mask, and visit Harry wearing the Arawn-mask... Or introduce Harry to the entirely-separate deity, Arawn. Etc etc etc...
Bonea does seem like an excellent source of this sort of thing! OTOH, I'm sure Jim needs her not to be Harry's
deus ex cranium. So there's probably a fair bit of stuff that Bonea
could credibly know, that she
doesn't know, simply because Jim needs her not to know it. Or maybe she "knows" it, but it's all uncategorized and she has no way to "remember" it -- maybe she'll spend YEARS on that project...