Despite the Savagekin class being non-canon, as stemming from the RPG books, let's take a look at what it says.
- Would be able to communicate be speaking or even telepathy?
As they are able to retake their humanoid forms, we can assume that they are still able to communicate with sophisticated language. However, they differ from druids, who can speak in animal forms, as they identify more with beast than with man. After a while, your savagekin will probably lose the ability to communicate with others of his or her own race.
Luckily, you can still chat with animals according to WoWpedia.
- Would I be able to change into different animals? Or would I be devoted and stuck as one animal.
As they are essentially "becoming the beast" they are transforming into, I think it would be safe to assume that past a certain point, the druid training is forgotten and one form is mastered.
- How long would I have to spend in forms in order to start becoming feral?
There aren't any solid numbers, but I think it would take a few years ranging up to decades of nothing but feral form with no contact with civilization.
- How would I roleplay as a actual saveagekin and not 'Dwah, look at the kitty kat or the teddy bear'?
You don't touch cities. Ever.
You are incredibly wary of strangers. Depending on how feral your savagekin has become, you can attack them with little to no provocation.
Don't "purr" if someone tries to pat you on the head. Bite their hand off.
As a savagekin, you have begun to abandon the societal restrains that your culture had forced upon you. Being nice to people is incredibly low on your priority list.
All I know is that it is frowned upon