It seems the translations for Avidemux are in a folder called i18n.
/avidemux/qt4/i18n
But if they are translations maybe the folder should be called l10n instead.
Maybe even better to have the translations in a folder in the root folder so its easier to find for people.
Maybe you can make the Windows installer translateable and find out where it would be best to put those files.
If not already possible i would like to suggest the Windows installer to be made translateable.