Once installed, language files for the plugin can be found in the following location:

administrator/language/<language code>/...

E.g.:

administrator/language/en-GB/...
administrator/language/nl-NL/...

Each language has a file for administrator and site:

administrator/<language code>.plg_content_mp3browser.ini   // for site
administrator/<language code>.plg_content_mp3browser.sys.ini   // for administrator

E.g.:

en-GB.plg_content_mp3browser.ini   // for site
en-GB.plg_content_mp3browser.sys.ini   // for administrator

Some languages available by default are: English, Dutch. But clearly you can add your own languages.