The White Chronicle is a small wiki dedicated to the game (with a mirror here) is a good place to start, you can also check the GameFAQs for the game as well. IGN also has a small FAQs page for it too.
You might miss out on some of the story, but it shouldn’t affect your actual “completion rating”. However, keep in mind that it may not be possible to get the skill before you reach the blocked point. At the blocked point, you may receive some item or information that will allow you to get the skill you need in the other timeline. You can’t find something ...
None of those games can be played with japanese audio in their english releases:
Radiant Historia has no voice acting so that wouldn't be an option.
Both Kingdom Hearts Days and The World Ends With You offer no such option.