I'm interested in using the n800 for collecting data in the field, but need to enter Czech characters. Is is possible to pick Czech for the on-screen keyboard? I know that a lot of languages are supported, but have been unable to determine if Czech is possible, as there are very few n800 units here in the Czech Republic.

There is no czech layout available but I think the format is documented on maemo site so you can create own layout. Currently czech characters can be entered but it is not very usable - there is extra '&!é' button and two accents tabs with all international accent characters

So, right out of the box I can do háčky and čárky (e.g. čříý), but the interface is not very easy to use?

I'm a programmer, so I'll try to make my own keyboard file to make it easier to input Czech. I just ordered the n800 today and will have it later in the month.

I just found recently added project https://maemo.org/downloads/product/OS2007/czkeyboard/

How difficult is it to make a custom virtual keyboard? I'm not a programmer and not an experienced Linux user but I'd like to use Hungarian keyboard layout on my N800.

I haven't taken a deep look but maybe this is a solution.
http://www.math.ucla.edu/~jimc/nokia770/scrawl.html
(majd ha lessz időm nekiállok megcsinálni, bár nemtom milyen lenne a jó kiosztás)