My preferred method of entering characters outside of the ASCII range (mostly European diacritics) is with the Compose key. My Compose key is the Pause key; this is configured by the following line in /etc/default/keyboard on a Debian system: XKBOPTIONS="terminate:ctrl_alt_bksp,compose:paus" So, to enter "í" for example, I type (and release) Pause, then the "'" key, and finally the "i" key. This works everywhere - in old style X programs like rxvt, in Gtk programs like Geeqie, in Emacs, in Firefox. But not in Claws, at least not in the message search input box I get by typing "/" when I'm browsing the message list. Entering "í" as above works, but not "á". When I try to follow the procedure for entering "á", I get a plain "a" instead, and to erase the "a" I need to hit Backspace twice. I suspect this has something to do with the extra keybindings that must be in effect when the search box is focused, such as Escape for hiding the search box.
Verified that it is present in 3.10.1 as well.
Key handling in quicksearch has recently (in 3.17.2) been reworked to not block unhandled keypresses. Please give it a try, and if this is still a problem, feel free to reopen this bug.