No, they're all related. Here's how the split works from what I know.
Games 1 and 2 are their own complete story. Game 3 is a sequel to games 1 and 2 with the same cast, but also sets up future plot developments in later games.
Games 4 and 5 (Zero and Ao) are their own story arc. It stands on its own, but there are some events in this game related to previous games.
Games 6 and 7 are their own story arc. They mostly stand on their own up until near the end of game 7, where characters from games 4 and 5 come into play (including straight up spoilers for some pretty major stuff). Due to the way game 7 ends the next game will likely continue this story arc and may bring more things from games 4 and 5 into play.
The third, fourth, and fifth games aren't being localized because Sen is more marketable with its more modern presentation and is available on "current" consoles. This is despite Zero and Ao being considered the best games in the series by most series fans that I know.