Well yes you can select your job but it all depends on your GT score. When I was in the max GT was 130. To be accepted to SF you had to have a 110 or above. But as I recall you had to be higher to get Com, Eng, and especially Medic... think medic was 120 or higher. I had a 127 so had the pick of the litter but decided to do engineer cause you get to work w/ explosives and the medic program took like 2 years to complete but when you finished you were a few classes and a test away from getting your PA in the civilian world. But when you get to your team everyone cross trains each other so you end up knowing all the jobs... well to a certain extent.
As far as language goes your GT score it highly dependent on that as well the guys at the bottom ended up w/ Spanish but as I had a high score I had the pick of the litter there as well and was slotted to go to 1st SFG which is the far-east and was gonna do Korean as I am half anyway. But as I recall, vaguely since it was almost 10 years ago, in ascending order Spanish, Russian, Arabic, and lastly the Asian languages of which Mandarin was the top tier and far as difficulty.
I wouldn't worry about the language and MOS as much as being physically and mentally ready just for SFAS. The rest will fall into place as you progress through the different stages of the course. Yes have an idea on what you want to do but you have to have the first step before you are taking any additional steps. My deployment in OIF 1 helped me to pass the course just by wearing and caring my flack vest, plates, water and ammo every day... you would be surprised how much strain and extra 45 lbs puts on the body when your wearing it like a shirt everyday. Its best to let your body get used to increasing weights.... and learn how to run and maneuver with it. If you can do some 2 mile runs with just a fack jacket and plates then add more weight as it goes on. I did a lot of quick sprints with a 80 lbs ruck over like 2 to 5 miles. Train hard but prior to going to SFAS let your body get a good rest will very minimal work outs for around 5 days so it can heal and when you hit the selection course you will be in peak condition..... just listen to your body and see how much time it needs to be back at full speed... some guys may need 2 weeks... some may need 2 days... everyone is different.