A really good resources to get information on best time of year and to be honest a great site to just help plan your visit is tdrexplorer.com. They have all kinds of details about visiting, best times, etc.. There is also a great crowd calendar site at
http://tdrnavi.jp/forecast/disneyland?lang=en that can help in planning your visit time.
When I was there for the first time last fall, with the exception of airfare I didn't really find it any more expensive than a trip to Disneyland. The biggest variable to cost is your choice of hotels. I spent most of my visit at Mira Costa which, while amazingly convenient is really expensive and I'm not sure I would choose it again for such a long stay due to cost. There are a number of non-TDR owned hotels along the monorail route that are just as convenient and much less expensive..
EDIT: I missed your question on language barrier. I do not speak anything more than basic greetings and had no language barrier issues at all visiting TDR. All of the attraction signage throughout the parks is in english and japanese and most announcements are also in japanese and english. Many live shows are also in english but some are in japanese. I was unable to figure out a common denominator on when english is used. When it comes to attractions, everything inside pretty much all attractions is in Japanese but I did not find that a barrier at all.