I think it's two reasons, which are related. First, I think the Universal translators on both ends of the communication, in addition to converting between languages, also convert between coordinate systems and units of measurement. This conversion has existed in our computers since the 70s, so it's not a big leap. Imagine the following scene:
Kobali Vessel: "We're 5 megaviotes from the meeting point."
Janeway on Voyager: "What?"
Kobali comm person ""does some math", we're half a light year away."
Janeway on Voyager: "oh. okay. see you in an hour."
That would get old very quickly for everyone. Based on this ludicrous exchange, I think all UT capable species in the Star Trek galaxy have auto-conversion of measurement and coordinates in their translators, just to avoid this matter. Why would the Borg be any different?
Other reason: we often hear people speaking English in scenes where it makes no sense, such as on Klingon vessels "I'm looking at you DS9". My theory is that we're not hearing the language that's used in-universe. We're hearing what is effectively dubbed dialog. It's easier than having the entire cast do the scene in Klingon, then subtitling it. Since the writers convert the language for the viewer's convenience, why would they not convert between units of location and measurement, for the previously stated reasons?
In summary, the conversion happens because in-universe it makes sense, and it makes sense to the viewer.