There's a Kuuga Complete Box you can buy that contains copies of the shooting scripts for the entire series. The shooting scripts usually contain rough translations of what various Grongi lines were supposed to be, to help the actors deliver them properly.
Grongi itself is just a phonetic substitution of Japanese and isn't a complete or consistent language. There are certain words that consistently mean the same thing, but for other lines the phonetic substitutions are pretty much whatever the writer thought would sound cool.
Overall I prefer the subs that just transliterate the Grongi. Using the script book translations throws off Kuuga's pacing, since the Grongi talk about certain plot things to each other long before the show "reveals" things to them to the audience.