Honestly, whatever firm I end up working at might be the tie-breaker when it comes to picking a language to learn.
If I end up working somewhere that does a bunch of appellate stuff and occasional SCC advocacy? Even more reason for me to learn French. A lot of the other firms I'm looking at? There'd be upsides to learning at least one of Mandarin, Cantonese, Punjabi, Hindi, and Farsi, depending on the firm.
