The queue line videos have subtitles usually, but nothing on the rides, though occasionally some shows will have some random English, eg Universal Monsters.
If i had young family I'd not hesitate to go to Japan first! It's closer, cheaper and basically same time zone, so no jet lag.
On the whole I felt it was an easy place to travel (so long as 1 person has a roaming data pack), its safe so it feels relaxing, and seemed kid friendly in many ways
The parks will be busy though, and you will get on everything at least once, but you have to carry the mindset that for every fast pass ride you do, you'll have a 90 min wait on a different one.
And of course, the holiday as a whole will probably be more interesting than say going to LA, and going to the parks + hollywood + santa monica or whatever people do on their first time in the US.