Celestial Pegasus
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If your Bard is the sort that plans to do a lot of summoning (I believe the common wisdom is "don't do it, play a Sorc or Summoner instead if you're that insistent on summoning", but the option is certainly there), then it should be mentioned that Auran, Aquan, Ignan, and Terran are useful for controlling Elemental summons.
Otherwise, the others have made the better suggestions already.
Callarek
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Kelish & Jistka can be of benefit.
Thassilonian, as mentioned. Sounds like it is going to be the go-to language for Season 4.
Tien and dialects for Season 3 (Ruby Phoenix is in Tien)
Ancient Azlanti (lots of places)
Ancient Osiriani (We are tomb robbers, you know)
Draconic for kobolds
Goblin for goblins
As mentioned, the language appropriate for the template attached to any summons you do.