From Nebelwurfer’s site:

Latest Patch fixes: All the Dialog Screens that contained Japanese text as part of the background image have been translated.

All of the scenarios in the Alternate History and American Front sections of the Campaign have been translated. This brings the count to 78 out of 103 campaign scenarios translated.

This should make the game playable. There is still some untranslated Japanese text that appears in the game. Mostly these are error messages that appear when you do something wrong. All the scenarios needed to play the campaign to a successful conclusion are now translated.

I estimate that the translation is 70% complete.