is someone could tell me why when I enter the composer's name in mp3tag and I listen to this track on Windows Media Player, the column where appears the composer is empty ?
Check how many composer fields you have: open the extended tags dialogue in MP3tag and make sure that there is only one field called COMPOSER.
Also, WMP has to update the internal database first. Make sure in WMP that the folder with the modified files is one of those that are watched by WMP.
To force an update in WMP, delete the entries from WMP's library (only) and then use File>Options>Restore deleted objects. This makes WMP to re-read the files and tags again.
It is a little strange that a single field should not be displayed - just to be sure: you did create a column in WMP to show the composer? WMP does not show the column of its own ...
Mistakes like typos in a field name are one of the favourites, e.g. Germans tend to create a field TITEL instead of TITLE.
If all else fails: use one or more programs to check the integrity of you your files. Good candidates are MP3val, Mp3diags and Foobar2000 (and the utility context menu functions).
And yes: I see all the composers I entered. And as I used the "semicolon blank" as separator, WMP even shows each name separately, e.g. John Lennon; Paul McCartney has individual entries for John Lennon and Paul McCartney in the list of composers.