Mozilla provides seperate downloads for firefox in different languages. You can download firefox for your language at https://mozilla.org/firefox/all.

i allready download and install the language but i still cannot change into english?? i restart still not change? how should i do?

You can't change languages from within firefox. If you want the english version, you should download and install the English-language version of firefox from the page listed above.

What locale do you currently have set if the check the general.useragent.locale on the about:config page?

You can open the about:config page via the location/address bar. You can accept the warning and click "I'll be careful" to continue.

• http://kb.mozillazine.org/about:config

Try this:

• Download the en-US Firefox installer and save the file to the desktop
  https://www.mozilla.org/en-US/firefox/all/
• Start the installation with a double-click on the desktop icon