I have a druid and I am big fan of the animal aspect (aka being able to talk to them etc.) When I picked my first two languages I picked Sylvan and Terran. I don't have much knowledge of creatures thus far in the game and do not want to waste a language on something I will never use. For a druid what languages would you recommend to start with if you have a +2 intelligence modifier? I thought Sylvan for Fey, and the others listed with it. I picked Terran so that logically controlling elemental's = awesome = follower. How should I optimism my druid's language capabilities? I picked the half-elf race so no language barrier.
Sylvan covers plant creatures and lots of other 'forest' creatures other than fey, it's a pretty good choice for what you're going for.
Terran on the other hand, is a bit rarer. Very little speaks Terran. In my games, I have underground creatures often speak Terran, aerial/flying creatures often speak Auran, underwater creatures often speak Aquan, and a disproportionate number of outsiders speak Ignan.
But RAW, you're looking at like, Earth Elementals, most of which don't talk to people. Summoned Creatures charge at foes automatically, so you don't need to talk to them usually.
Language choices - Treant is a language in pathfinder, or as I like to call it, Entish. That would work for a druid. Unlikely to come up though.
You already speak Common and Elven, but Dwarven is the other traditional trifecta of languages, although nearly all GMs will never have it be important.
Undercommon covers probably the broadest category of races and situations, but again most GMs won't have it come up.
Surprisingly large numbers of common PC enemies speak Giant, and ogres, giants etc are all 'wild' humanoids, they mostly don't live in cities.
Dragons and dragon-related things are pretty common, and Draconic is also (apparently) the most common language of arcane magic (sort of like Latin in Ars Magica).
Just have a look here, and pick something interesting. Mention to your DM that a big part of your character is that your character considers himself a linguist, and the DM might get ideas about working that into the story.
Summoned creatures that you can't communicate with won't do anything except attack your enemies:
So it's worth thinking of what type of entities you want to summon for other than direct combat purposes... although check if your DM is actually going to use this rule, since many don't.
Terran is actually more useful than the number of creatures that speak it implies. The earth elemental's earth glide ability makes them amazing dungeon scouts, but only if you can command them and understand their report!
From an 'optimization' perspective, the best choices for languages are the ones which your summon creatures use (so they can understand your commands). As Jack said earlier, Terran is a rare language to encounter while adventuring, but Earth Elementals are amongst your best summons.
For this reason I would recommend getting Terran, Ignan, Auron and possibly Aquan fairly early on.
Later you want to definitely think about getting Sylvan and Giant (as high level SNA have a lot of fey and giants in the list)
Aside from summons, the rest of your choices should depend on where the campaign is taking place. But it's probably best leaving those encounters to classes that can cast comprehend languages.