[Shasown]

Early written languages were done using runes, its from these runes most (but not all) modern letters are derived, however there will always be differences in languages because of local factors, the difference between English and Irish alphabets can be explained by the fact of both Roman (Latin) and Saxon influences on the English language as opposed to the almost pure original Irish.

Old English as in Celtic would have been very similar to Irish/Scottish - Gaelic. And Welsh - Celtic.