Atai is not only a highly informal term, but also isn't even a proper word: it's a slang version of
atashi.
Since English first person pronouns make no distinctions on the gender of the speaker or formality levels, both the female-only
atashi and the informal
atai have no proper translation. So, the only thing translators can do to come up with something different than a generic "I" is to either make some slang out of it ("ah'm", maybe?) or to alter its writing ("eye'm"). They seem to have decided on the second choice.
I once found an alternative translation for
atai in
this picture. Personally I like it a lot; "if I" sounds a whole lot more like the original "atai" than "eye'm".