Here, in Europe, we get a lot of video sources with multiple languages to select from. I edit those regularly, and always lose the multiple languages. The documentation says:
"By default, Kdenlive creates a stereo audio track. Use this option if you want multiple audio tracks (for example for different languages or commentary) that can be selected in the player software. Use a third-party software to put the video and audio files together."
While this works, I would much prefer to do the same within kdenlive. Including more than two languages, b.t.w., and not having to resort to 'man ffmpeg' ever again to reconstruct the respective command line.