So...
I am really liking zero.
Which means I should probably play Kiwami next when that comes out yeah?
After that, would i know enough to go to 6? Is it worth tracking down the other ones that have been translated (which one are those even, can i get a translated 2?)
Is the gameplay hard to go back to at this point?
every mainline yakuza game has been localized. Including 2. The gameplay is not hard to back to because the games really aren't that old, remember the whole series is only a decade old at this point.
however, tracking down 2 and maybe 3 will be more difficult than 4 and 5 since the latter are available digitally and 2 had a low-print run.
really, i wouldn't worry about it, you'll be fine to play 6 after kiwami. if you want some yakuza before that comes out, 4 is really good (i used to recommend it as a starting place before 0!) and is available digitally.