Flak Attack actually contains the Overseas game, without the version menu that's standard for all other multi-region ACA releases, EG Salamander/Life Force.
It turns out that to enable Flak Attack's region switch, you need to enter the Konami Code at the ACA title screen. That's [up up, down down, left right, left right, X O] - here's a GIF:
Spoiler
This was driving me nuts yesterday. While I'm unsure of region differences between Flak Attack and MX5000, Konami tended to screw up their Overseas versions (cf Thunder Cross, XEXEX, Haunted Castle et al... see also lesser inferiorities, like Super Contra's missing second loop). I actually deleted the game and re-downloaded it, only to have フラック・アタック become MX5000 upon completion.
I wonder if there are any similar cases. I kinda hope not, because it's rather too important an option to hide away. Anyhow, all's well that ends well. I figured this out only by the grace of finding our own GSK's article at Retronauts - thought I'd post it here for posterity.
Always hated that name. Still, game's the best of the series by a million miles. 
(three million, if you count the other two characters!)
) and order, even. Twinbee plays its st1 BGM, though without the "music box" presentation. Gradius and Salamander play an unused BGM each (AFAIK - I don't recognise either tune offhand).
