Maybe it was left in to ensure people knew he had it? Since things like that don’t really exist in English it was the only way to show he had a speech pattern (other than a note of some sort). Even the translators for the manga kept China’s ‘aru’ in and added a note as to what it meant somewhere.
Most of this stuff is easy to interpret if you know some Japanese and recognize it as a verbal tick but not all of them have been explained to the English fanbase as a result, so people use them thinking they mean something, not realizing a lot of those ticks only work in Japanese. In English he’d probably just have an accent or something, but I don’t know if i’d want to read an accent written out.