Dogen and Lennon have both received the same blessing of agelessness which Jacob bestowed upon Richard Alpert. This seems to match Lennon's very "retro" appearance despite his apparent age.
Role within the Others
Lennon is Dogen's superior, and his translator role is just an act. Dogen does not need translating to because it isn't him that needs to know anything (as well as that he understands English) - Lennon is merely giving himself time to think before giving the English answer (ie - his own decision). This would fit into the "Man behind the curtain" theme.
This seems to be debunked by the fact that Dogen's Japanese is being accurately translated.
Okay, this show has a history of characters being named after real people, and usually these characters bear some resemblence to the person they have been named after, belief-wise or personality-wise. This guy is clearly named after the Beatle, John Lennon. What do you think that signifies?
"Imagine there's no heaven; it's easy if you try.
No Hell below us, above us only sky.
Imagine all the people living for today.
You may say I'm a dreamer, but I'm not the only one.
I hope some day you'll join us, and the world can live as one."
Alternatively most of the namesake characters are named after philosophers, scientists, politicians, etc. Perhaps, although his name is spelled Lennon, he's actually supposed to be named after Lenin.
The glasses that he uses seems to suggest pretty much otherwise.
He is in fact the former Beatle, John Lennon, who somehow survived his apparent death in 1980.
He is not John Lennon. An homage to him, possibly. But it's not him.
In Lost, everything is possible
His close pairing with a strong Japanese personality (Dogen) might be another homage, and the original reason for it.
Unlike the real Lennon, he doesn't appear to be 66/67 (assuming he isn't now ageless). As an Other, he also appears to hold different ideals than John Lennon, not really minding when ordered to kill a group of wandering strangers.
At least we know who's infected Sayid now: it's Mark David Chapman.
Lennon is no one of any particular importance. He isn't connected to Dharma or one of the Losties. He doesn't have a cool backstory. His lot in life is to be the translator for a guy who doesn't need a translator.
Based on his appearance, Lennon was original a member of DHARMA before becoming an Other.