I assume that you are using the AppImage version. That version is English only.
Your options for other languages are either to build Audacity from the source code, or wait for your distro to update the repository version, or look for a working PPA build.
I followed this up, and it appears that the AppImage was intended to have the full set of translations, but for an as yet unknown reason, it is English only. In other words, it is a bug.