I am a a korean. This is might be exact translation what you asked about. 나는(저는) 너에게(당신에게) 나(저)에대해서 말(얘기)했던 사람이야(사람이에요/사람입니다)

Wow, Mojave's answer is fantastic. kkk, i don't believe he is non Korean.

In Korean, (and, of course, I am not a native, just a learner too), there are two parts to the sentence: "I am the one" = 저는 ... 사람이에요.; and then you want to explain which one you are: "the one John mentioned" = 존씨가 언급하는 사람. So, "I am the one John mentioned" = 저는 존씨가 언급하는 사람이에요.

In English, you should say "I am the one John talked to/told you about." - "me" is redundant and awkward. Technically, you should not end sentences with prepositions (although it's pretty common these days), so you could say "I am the one about whom John talked". But I think a better, more natural verb is "to mention". "I am the one John mentioned."