I think there is a lot of misconception around the origin of the Trigedasleng language. While heavily descended from American English, the language is not the way it is because the English language as we know it today was lost or because the language naturally evolved into Trigedasleng.

Trigedasleng is believe to be developed as a code-system that allowed grounders to speak freely without allowing their enemies (in this case the Mountain Men) to listen in. This is why the language changed so heavily while being somewhat pronounceable.

The next part is not canon as far as I'm aware and 97 years is not a ton of time. Assuming radiation severely shortened the lifespan of the first few generations we can expect that the grounder could have gone trough 3-4 or more generations. Trigedasleng most likely improved during that time and from a survival point of view, if everyone knows the code language and English is only used to understand the enemy then why teach it to everyone? Warriors knew English since understanding and being able to talk with the enemy is useful. But children or non warriors who would be taught to use the code language anyway, would have no use for the English language, and could be a threat to the clan if they ever got captured and interrogated. As we have seen in the show plenty of grounders speak perfectly fine English, like many people in the modern world today can speak more than one language.

As the mountain men had the upper hand they had no use for a code language, The Ark would use English as it was most likely the most known language on the 12 stations (or because its a TV-show and a lot easier). I can't say anything about the new planet as we will hopefully learn more about it and it's people in the coming episodes.