Unfortunately, this is a bit difficult. Decker and Lil do not include Unicode support, as it would add considerable complexity throughout the application, especially in the text layout routines and the fonts shipped with with every standalone deck. This was a tradeoff I made to keep the scope of the project within my own limited means to develop it.
It is possible to create custom fonts that remap ASCII characters to a different set of glyphs- this deck, for example, uses a customized font to display some text using the Cyrillic alphabet, and I know of at least one person tinkering with a Hiragana font. Rich text fields support inline images, which offers a (somewhat clumsy) solution to needing a specific additional character or two. The biggest downside to the customized font approach is text entry: Decker has no way of knowing how to map non-ASCII characters into the glyph set of a given font. Depending on how different your language's character set and/or keyboard layout is from US-ASCII, it might be very unpleasant or confusing to type.
I apologize for the accessibility problems posed by these constraints. I think most improvements would be highly language-specific and possibly quite daunting in scope.