Jack's tattoos don't actually mean "He walks among us, but he is not one of us." In fact this is nowhere near correct. A word by word explanation of the four Chinese characters on the tattoo: The first character means "eagle". The second character means "strike", as a verb. The third one means "long". The final one means "sky","space","void","empty". These characters are pronounced "ying ji chang kong" (in PinYin) These four characters are taken from a famous poem written by Chairman Mao Tse-tung (Mao Zhedong) and can be roughly translated as "Eagles cleave the air". But the poem itself has a deeper meaning. In the end of the first stanza, the poet asks, "who masters fate's rise and descent?"
The tattoo that Jack gets in Thailand is actually a real tattoo on actor Matthew Fox. It is covered with makeup in the flashback scenes, but this is obvious as the color doesn't quite match his skin tone.