SQUiRE have released a language selector for yesterdays XPA release of Metal Slug 7. In order to attach the selector to the game you will need to use bspatch (download below)
Just tried with CycloDS works perfectly. Also it's a much better hack than using the standard way of renaming the files in the rom and repacking. The text doesn't wrap in the SQUiRE hack and the pointer shows up in the combat school when selecting yes no etc. That was missing for me when i manually hacked it yesterday.
Wow even the pause menu is in English and the hostages names. 100% English by the looks of it.