I think for native speakers it is difficult to understand what I mean.
For you, English is the one you know and the best you know. For me to give an example sounds self-explanatory to you.
However, I have handful of examples, but even those, may seem weird to one who speaks it every day.
The entire quest, for example Ivalice, I understood not a word, I had also huge issue with Weapon quests, the story's way of explaining technical things, the theories formed in such complex words after words that when you look up definition it becomes harder to put two and two together.

Again and again I am told the English is 'straight forward' but it is truly not for many, for instance, from Europe, or otherwise Oceanian outside AUS/Zealand.

It is the nice example you thankfully have used with the child NPC in the seasonal event too.
Another example which was HUGE, it took me HOURS before I finished it, it was in Amaurot. The NPC debate, the answers, the talks.
My head turned, I even had to take long breaks. It took me not only hours each, it took roughly a day and a half, because it was so much difficulty to understand, and I wanted to understand everything. However, I have less issues when it is cutscenes. If it makes sense? ^^