So I summon a celestial dog. Can I tell it who to attack or where to go or what to do? According to the summon monster spell description it seems that I can't as I don't have a way to communicate with him. Is that right? So it just attacks and does whatever it wants?
Also what about Handle Animal skill. Can that help me? And what about spells like Speak with Animals? If I can someone get a scroll / potion with that effect, that should help, right?
Ok you missed a few points in the spell description so I'll point them out and it should clear up your confusion:
"This spell summons an extraplanar creature (typically an outsider, elemental, or magical beast native to another plane). It appears where you designate and acts immediately, on your turn. It attacks your opponents to the best of its ability. If you can communicate with the creature, you can direct it not to attack, to attack particular enemies, or to perform other actions. "
Ok so the bolded part is key and not language dependent. It appears and attacks your opponents to the best of its ability. It will do this even if you can't communicate with it. Note it does it to the best of its ability so you wouldn't need to direct it to use combat specific abilities such as trip or grab.
IF you want the monster to do other things though, like fly through a window and grab something off the table in the room you have to be able to speak to it. If you wanted it to not fight but instead take someone off in a different direction you would have to speak to it. It will generally attack whatever opponent is closest unless you can direct it to attack a specific target by speaking to it.
If it doesn't speak your language, and you don't speak its, then yes.
If it's an animal, yes. If it's not an animal, you might need another skill.