though I learnt they're switching to scala
They're not actually switching to Scala. They're porting a large
portion of their computational work to the JVM with Scala being one of
the languages they use to power that.
The web interface, last I checked, was still running on Rails :).
I'm currently learning ruby as a beginner to later carry on to Ruby on Rails with the sole aim of developing a website similar to twitter... the project is a bit complex than twitter and better.
I'm glad you're learning Ruby, and looking towards using Rails. I feel
I should point out that any Twitter Clone (and there have been many,
like RStatus) really can only "beat" Twitter by becoming bigger.
That's harder than simply writing a few thousand lines of code (which
they've done as well).
What I want to find out is if I can build and complete the whole project with only Ruby on Rails as the framework and obviously html/css as the design structure?
To answer your real question: Yes, in fact Twitter did it themselves!
You've even got the advantage of having a more mature Rails. That said
the reason they're replacing large portions of the process with Java
related architecture is because Twitter handles *a massive amount of
data*. It requires quite a lot of power to handle the kind of usage
statistics they bring to the table. As a result they needed a language
that was closer to the mettle, faster, and pretty stable under heavy
loads. This isn't to say that Ruby *or* Rails are unstable, simply not
as fast as Java.
In the end you'll have to become somewhat of a polyglot, or at least
have friends who know languages that do things better than Ruby if you
want to make a "better" Twitter.
Keep on your path though, and remember that the Rails Hotline is a
free and open source of (volunteered) help for Rails developers.