I have prepared an MPlayer package  that has on the fly i18n
support. MPlayer now checks for the LANG environment variable to select
which language to start with and also passing a "-msglang nl" like
parameter will force MPlayer to start with that language. This way
MPlayer frontends that do string parsing to understand what is going on
will make sure MPlayer is started with the language of their choice.
Please test with different locales / LANG env values / MPlayer
frontends and let me know how it is working, what is working right,
what is working wrong etc.
PS: I am aware of some gnome-mplayer glitches that are not related to
i18n change, but it won't hurt to let me know about them again.