What motivates me to learning English

I have loved learning different languages since I was a child, but the last time I've studied English was when I was in school, so my Englisgh is a little bit rusty.

you could also say "the last time I studied English was when I was still in school" or "I haven't studied English since I was in school". Both sound more natural.
Use of "a little bit rusty" is perfect here

It's true that I've been learning French and German, but I've forgotten the basics of Englsih, so one of the main reasons that motivates me is, actually, to remember this beautiful language (it's a pity that learning languages isn't the same thatas riding a bike - I haven't rideden a bike for years, but I remember how to do it!).

This is a complicated sentence but you made very few errors - that's great!

Furthermore, I currently live in a city where everybody speaks in English, and I'm very ashamed of my lack of oral fluency in the talks (I can't communicate well with new people, and I'm looking for making new friends!).

You could also say 'I'm trying to make some new friends"

Finally, my field of study and work is international, so I need to communicate with people from all over the world, and obviously I requireneed to speak, write and listening to this language every day.

everyday is an adjective "everyday clothes" means basic, casual clothes. If you mean something you do each day, it's two different words.