Spell doesn't require that it speaks or understands any of the languages you do.

yes, that is a very good stratagy

I think that whenever it burrows is when it uses earth glide, as it is in the traits section, and not the actions.

Meow Meow Meow Meow

Guessing that the bludgeoning damage from the Earth Elemental is considered non magical?

I don't see how a level 5 party could survive an encounter with this beast. Slam, slam, burrow - full cover. The only way they can inflict any damage on him is through opportunity attacks. As long as he always picks out a single target it will be two slams VS one opportunity attack. He'd demolish a party of 4 easily

If u know primordial u speak terran

My Dragonborn Fighter has one as a pet. It's name is Noibert. Why Noibert? IDK

in the book series drizzt do'urden the duergar captain used an earth elemental to attack drizzt. maybe more info in there?

Quote from Oneadventurer >>

Am I crazy or can an earth elemental make its two attacks then sink into the earth for full cover?

 I know this is very late (literally 3 years), but I believe it would probably take an opportunity attack from that, and if not, it only has so much movement, so it could do that once, you run 30 ft away, it burrows 30ft to you and pops out, but cant go back in.
Alternatively you can ready an action of attacking them when the elemental pops out of the earth.

I believe large is 9ft-16ft, so it couldn't be 8 feet, but it could be 9 I suppose (you might want to find some sources backing that up though because I feel like there's probably a height that these are usually at).

magic 

Quote from Lomonocrat >>

If they only speak Terran, how will an Earth Elemental understand my half-orc's verbal commands when I summon one with Conjure Elemental?

 It’s magic 

I was wondering if my campaign is starting at level 10. And I start off as a Moon Druid, then go 2 levels of paladin. Could I wild shape into a Earth Elemental and then smite? The image is awesome, and it says melee weapon attack. But I wasn't sure if that is pushing it.

Remember folks you can't use hold monster on this guy since he's immune to paralysis.
Unlike most other primary elementals, it can be grappled, restrained, or made prone, but it's still immune to paralyzed and that's what hold monster does.
Feel free to homebrew this to not be true though. Monks can punch it and stun it. 

Magic

Quote from Oneadventurer >>

Am I crazy or can an earth elemental make its two attacks then sink into the earth for full cover?

 If you give it haste, then yes.

I believe movement is an action in of itself

Magic.

I wonder if it can Earth Glide to find underground caverns then burrow it's way up to the surface to allow players to access it.

I mean i would scout and tell you everything... in terrant but still it did it.