Using
avidemux, load the file, choose copy for both audio and video streams, then select the desired audio language source from the toolbar. Then save as a new avi.
That should take care of it.
Alternatively, you could install
k-lite or
sharky's codec packs which will install the appropriate splitters and codecs to allow you to select the desired audio track from a systray icon, as well as enable support for other A/V formats, like ogm, mkv, multi-channel ac3, dts, pcm audio, subtitles, chapters, etc, etc...
If you're not interested in installing a whole bunch of codecs and other stuff, you could just install
haali media splitter and that should take care of it. I do highly recommend the k-lite codec pack (both x32 and x64 versions) though for all the extra format support.