If you are talking about an activity that is no longer going on, you want the simple past tense, also known as the preterite: I studied the Russian language for three years, and during that time I learned an astonishing number of words that start with the letters Х and Ё.
If you use the past continuous tense (I was studying the Russian language, and I was learning how to cuss), we only know that you were doing that thing in the past - not whether you finished or stopped.
You might, if you wish, include the duration in the same sentence, but in that case the sentence sounds very strange if you don't reference the duration in the second part as well - I was studying Russian for three years and I was learning to cuss the whole time. Even so, that sentence would sound better without parallelism: I studied Russian for three years, and I was learning to cuss the whole time.
You could use the present perfect (I have studied the Russian language for three years, and I have learned to cuss like a моряк on shore leave.), but there is no indication that you've stopped - in fact, the assumption will be that you're still a student.
The past perfect could work for your purposes: I had studied Russian for three years, and I had learned to describe my friends' ancestry in detail. Once again, however, we only know that (at whatever time in the past we're talking about) you had already studied for three years - NOT that you had stopped. You still might be studying today.
There are a few other options, but I think those are the most common.
To amplify @John Lawler: when referring to русский язык, you can say the Russian language or Russian interchangeably, but not "Russian language" or "the Russian". And "learnt" will be understood by Americans, but will definitely look exotic.