depends whether he can read Runic?

there is nothing special about Latin as a writing script for spells - Hermetic Magic does not use "magic writing". Greek, Hebrew & Arabic spells are known, also spells in non-Latin languages using one of the other scripts.

If on the other hand, you feel that Norse Runes have an innate magic in them, then feel free to expand this in your saga. I would suggest that Hermetic Magi be very cautious using innate magic from alien magic systems!

To add to that Caribet, Latin is the language of the Order of Hermes, much like French is the language of most of the courts of Europe, just like English is a widely used language in commerce today. Everyone in the Order knows Latin, and so the Order can maintain unity despite how far flung it is.

Regarding the Runes, Hermetics undoubtedly employ Runes of various kinds in their spells, but that is covered by Magic Theory, not linguistic ability (covered by Latin and Artes Liberales). Which is why a person that tries to copy a Magic text, but doesn't have Magic Theory, will reduce the Quality of the text. And further, if you have no Magic Theory then you'd have virtually no chance of translating effectively a wizard's untranslated Labtext. This isn't a function of Latin or Intelligence, but knowledge of the Magic Theory.

So what would happen if a Magus put Runes into his book? Likely nothing. Unless the Runes themselves actually had power in them, they are merely symbols for the conveyance of power, which require will to employ. Verditian Wizards use Runes all the time in their magic items, but the Runes themselves are worthless without the techniques that put them there. Unless it's literally a Rune that must not be repeated, it should be fine. Such a rune though would obviously be enchanted just by the process of producing it.