How did you become

I think both sound correct, but where I live, we tend to say "about" instead of "around".

I am a beginner, so I'm not 100% on this, but I think dei is di+i and degli is di+gli. Gli is the plural of lo, the masculine article used before s+consonant, ps, gn, and z (and there may be other cases that I'm not aware of). So you'd say dei paesi and degli studenti. Native speakers, feel free to correct if I'm off here!

Maybe it's good motivation to study more!

Hi Ewelina!  Leaf is a singular noun and leaves is a plural noun.  Belief is a singular noun and beliefs is a plural noun.  Frequently in English, the noun is made plural by adding the letter s.  So 'leaf" is the exception, not "belief".  Believes is a present tense third-person singular verb:  "He believes that he is correct."  It corresponds to believe: "I believe I am correct." (First and second person singular and first, second and third person plural verb.)  I hope this helps!

Can you please help me correct this paragraph?

For this mobility programme I have chosen the Al-Farabi Kazakh National University in Almaty, Kazakhstan, mostly because I am aware of the fact that the Al-Farabi Kazakh National University has recently demonstrated interest in including Portuguese as a Foreign Language in their range of foreign languages available to  programme of study. By participating in this mobility programme, I would be able to give the institution the opportunity to offer Portuguese Language classes (provided that they would have me as a teacher for a period of six months) and I would also be given the opportunity to gain more experience as a Portuguese Language teacher abroad. While teaching at the university, I would also work on my doctoral thesis. I have been collecting oral data over the past months and they are it is still to be processed, hence I could use that opportunity to process them it and continue writing my thesis.
Furthermore, I have chosen this institution also because of the fact that I am very much interested in Central Asia and, more precisely, in the countries that once integrated  comprised the Former Soviet Union. I would like to have a chance to experience life in Kazakhstan, as well as to improve my Russian skills (gained through a four month stay in Russia in 2010 for study reasons), acquire a basic knowledge of the Kazakh language, and explore this fascinating corner of the planet Earth.