Japanese font Sizes aren't that large compared to some games. The Windows Font that contains Most common Japanese symbols are 8 MB or bigger. (Most fonts are collections that make fonts bigger, Ooki.org state one is 7.45MB
http://www.ookii.org/software/japaneseinput/font.aspx) It's also worth Noting that most of the Fonts have a Latin-based Letters already built in.
You can limit the size of the Font file, such as limiting Kanji. But that doesn't work well for a game already out in Japan.
Some PC game can do this ease.
That was the good news.. Here the bad.
There is a problem is that English Text is Taller than Wide.. where Japanese is roughly square.. (Actually 4 is square: 四) Also you can't fit in Japanese text in a single byte. So Japanese text can take up more space, in both in memory and on Screen.
In Japanese you can write a Popular Monster's name in 5 symbols, in English for the same sound, you need 7 letters. Most English games will alter the data tables, to make that you can have longer text boxes.
I know this sounds weird, but some Text may not be actual text but images, or worse, a 3d Model, and you'd have to tell the program to change images, or Models depending on Language settings. That could be difficulty if this image/Model is part of the "world," as you need to alter code that wouldn't normally be touched. Hey, If you're to find a message in the Landscape as subtle part of the plot, you really don't find the Clue in the wrong language, particularly if you needed to translate it for localised version, and can't change it.
http://tardis.wikia.com/wiki/Planet_One (OK, the Tardis Translated that one.)
So far the only game that was released in Europe with Japanese Text and English Audio was Sin & Punishment on Virtual Console. They did Translate all text except for the cut scene subtitles.
Here is a Old interview.
http://www.ffcompendium.com/h/interview.shtml This was done soon after the Release of FFX-2. With another interview linked to it.
An Interview about a Prinny:
http://www.siliconera.com/2010/04/28/ni ... ns-part-1/ and
http://www.siliconera.com/2010/04/29/ni ... ns-part-2/Ok, Here's the Deal, with Not having English Audio, when subs are available. It cost time and money to ask Voice actors to come in to record the Audio. The only games I've fond this are RPGs that use little Audio such as Kingdom Hearts: Chain of Memories GBA, it used already recorded English lines for the returning Characters, and Some fighting games, such as Naruto Ultimate Ninja 5, which would need most characters to record new lines, Plus the fans like the Japanese Version due to a CrunchyRoll.
It's common for some European RPGS, to just to use the English Audio. Tales of Symphonia just altered the text to the Language, but keep the English Audio. There is a something that I've not checked, the German Version of Kingdom Hearts Translated from the Japanese text, but kept the English Audio, and in one scene the text said one thing, and the Audio said another.
As for NISA.. They know that the English Audio is high quality, and usually worth every cent they spend on it.