Click Options -> Interface Language to display the language selection dialog. In this dialog all installed interface languages are listed. Using the drop-down box, select the language you want and press OK.
If your language is not listed here, you can create the required file yourself. It is quite simple: just use the file english.lng (located in the program's folder) as a template and translate the necessary words and sentences from English into the language of your choice. The file has to be saved (in pure text format) as yourlanguage.lng (e.g. esperanto.lng).
Just three notes here. First, in english.lng, don't translate the words in square brackets (e.g. [Translations] and [Messages] - those are the "standard" headers which should not be changed), nor the lines to the left of the "=" sign because the program uses these strings as an ID of the translated text. Secondly, try to keep the sentences as short as they appear in the English version, otherwise they probably will not fit in the dialog boxes -- in some cases the program may even crash if the lines are too long. And last: don't change things like %s, %d or 0x%08X – these are the format specifiers.
Also, the "&" character must not precede the same letters in different words on the same user-interface screen, otherwise the keyboard shortcuts will not work as intended. If it does after translating, just put it in front of some other fairly distinctive letter in the translated word.
By the way, don't forget to inform us that you have created a new interface – we will include this file into the next version of software. If you contact us prior to starting the translation (to ascertain that you are the only person who is doing this work), you can receive free registration!