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Both translations are correct. And you could also translate "(My) father is joyful". The crucial point is the letter I after "av". Vowels don't usually matter a lot in Hebrew but this one does: it can be either a suffix indicating a possessive pronoun ("ach" means "brother" and "achi" is "my brother") or else it can mean - nothing, because it's just the relict of an ancient grammatical phenomenon which has lost its meaning. We just don't know.
Andy ;—)