Lynxara
Nice post!!
One of the reasons why there is some point that I got bored with Kuuga is that the Grongi language is not translated. Subbing the Grongi language is a good thing especially for new fans.
The villains discuss things in their "translated" dialog long before the audience is supposed to know about them. If you translate all of the Grongi in your fansubs then you are basically going to ruin a pretty big chunk of the show.
The Grongi scenes do drag on in some episodes, but it'd be better to just skip them or watch on 2x speed than to add translations that weren't published until after the show's original broadcast.
There are no Japanese subtitles on those scenes because you aren't supposed to know what's going on. If you were, Toei would've added subs for Japanese viewers during the initial TV broadcast.