@Yurnero "umu" is something like "understood", "uh huh", "got it", but it is normally said without words. Like mumbling to yourself that you understand. It's not directed to anyone as words but to indicate that you understand the situation or explanation with a vocal sound. There is no "word meaning" for "umu" since it is a spelling of what sound the mouth is making. Same like there is no word meaning for "hmmm", "ohhh", "mmmm". These are expressions of the mouth to indicate some understanding. For example, "hmmm" can indicate "contemplation/wondering", "ohhh" can indicate realisation/surprised, "mmmm" can indicate "thought process/decision making".
"umu" is only seen in Japanese translations. Probably they make those sounds in Japan with their mouth.