Some spelling & grammar adjustments:

Cohesion and coherence... I need some advices how to improve the text ;))

Part of my Motivation Letter. Could you help me to improve it?

This statement is to show the main reasons to apply for the academic mobility program at the HSE Spb.
Having certain academic achievements, foreign language proficiency (both in English and German) and academic international experience, I can certainly say that I am interested in further development in my professional field of Political Science. I believe that an international mobility will help me to develop both my personal and professional skills, and prepare me for academic development at the stage of the MA program stage and for future PhD courses.
I have chosen the following universities from the list of partner universities, as follows in order of decreasing priority:

1. University of Bremen (Germany)
2. Université Lille (France) (if you want to write it in English, then use: Lille University)
3. University of Helsinki, Aleksanteri Institute (Finland)

To be honest, my priority place to study is Germany.
The most important reason for applying to study a semester in Bremen is that I already have a strong vision of the research project for my future Master's thesis which involves Germany as a part of it.
<...>

Another factor that motivated me to apply for the academic mobility program to Germany deals with German culture. It is important to mention that I have competent skills in German language: in the 11th grade I achieved a Sprachdiplom. The acquired experience allowed me to absorb (?) both German culture and language.
<...>

My second priority university is Université Lille. My decision was guided by two main reasons:
First, because of a number of courses which include strong exploration of comparative research methods in political and social science. Secondly – it is a great opportunity to improve my French-speaking skills, my third foreign language, and to get new insights into French history and literature.

I have put University of Helsinki in the third place because of a common knowledge in my department (where I am currently studing at) about the quality of the study process at this University. One of our lecturers is also working in Aleksanteri Institute at the moment and he might in some sense help me to adapt in moral and research nethods at the new place. During my studies at the University of Helsinki, I will also get a tremendous opportunity to find and communicate with some famous researchers in my subject field.

Try to avoid contractions in a formal letter (use "I am", not "I'm")

Good luck!!