Xterm surely does not use any "languages"
shortly said there are mainly two kinds of "language environments"; command-line locales and your desktop environment's. You can set several locale settings for your command-line envionment, shell, like LC_LANG (language), LC_CTYPE (to get different chars than your LANG supports), LC_ALL (the whole thing) etc. These should apply to things started from the command line, and can be forced with, for example,
which should start 'irssi' with finnish locale settting for keyboard/characters, despite the language used in the system (and other locale settings). Then there is your desktop environment, for example, KDE. It uses some languages which might differ from what your locales are; I guess you should check out your locale settings, which locales are configured on your systems, which one are the defaults etc.
shows your locale settings and
all available locales.