Ori's writing

[Galin]

But Gandalf 'pored over it [the book] for some time' and had guessed that 'the pages seem to have numbers referring to the years after their arrival.'

Meaning Gandalf didn't just open the book, by coincidence, to the right page.

Translating Westron into English for the book of Mazarbul is in step enough with translating Westron into English for the Red Book or its copies. The 'problem' is, once you show a document in an illustration (which Tolkien at least planned to do here), the implication, at least, is that one is looking at a depiction of the real thing...

... but JRRT didn't invent enough Westron for that in any case, nor even the real Mannish names for the Dwarves that appear on the Doors of Moria for example (or Balin on the tomb) -- thus the illustrations are to give a 'sense' of the original, real thing, with respect to which the runes and Elvish letters help visually.

Of course in The Hobbit Tolkien, in theory, went the extra league when using his Anglo-saxon like runes there... I believe Tolkien waffled a bit about that (how to explain them), but finally landed on the conceit of the translator.