My first writing

Really well written and the meaning is clear, just a few minor alterations to make it sound more natural.  The words to take out are underlined and the suggested alternative in yellow (apart from the please that for some reason refused to be yellow)

Rio, I like what you wrote. You should post more on italki! You're good at it. Your introduction sounds quite natural already. But I'll see if I can help you make it sparkle!  

My first writing

I have been studying English for over a decade, and I am also interested in learning other languages. ***
I often listen to music in Spanish, French, German and English, and love watching TV series with while following the subtitles in English subtitles.
There are many artists**** I love and in future posts I will write about some of them, including: Charlie Puth, Ed Sheeran, RIDSA, Black M, Alvaro Soler, Daddy Yankee, Don Omar, Becky G, Anitta, SDP, Sido and so on. Oh... I cannot write all! Too many to name.

If there is someone who currently lives here and happened to enter this Website, don't hesitate to say hello to me :)*****

* I'm not sure this first sentence is necessary. In Japanese, it would be very natural to say みなさん、こんにちは！But in English, I think it would be more natural to just dive in with "Welcome to my Website!" (and because that serves as a title/headline, you should capitalize the W in Website).

** What Teresa suggested in her correction is a good option. And the definite article "the" is needed, because the "field" is a definite entity. But if you want to be more specific and tell your specific role, you would use the indefinite article "a" (because there is only one dental field, but there are many dentists, hygienists, lab techs, etc.). It would be nice if you told more about  your profession (" . . . working as a sales rep for a dental supply company" or ". . . working as a hygienist in a family dental practice" or whatever)

*** The "also" in this sentence makes sense, but I think the rhythm is better without it. It's up to you.

**** You could say "musicians" or  "musical artists." It would help make it more clear, because you went from "music in Spanish, French, etc." to "TV series" and then back to artists. It's a little confusing.

***** I'm not sure what you want with this last sentence. Do you mean that is a visitor to your site lives in Iwaki  or Fukushima-ken, they should say hi? Does that mean you don't want visitors outside of your area to comment? Maybe a better way to conclude is:

"Thank you for visiting my website, and be sure to leave a comment. If you happen to live in the  Fukushima area, be sure to let me know."