If you only want to use the English version, download the English version of Firefox from http://www.mozilla.com/en-US/firefox/all.html and install it on top of your current version.

If you install on top of the current version then you need to reset the pref "general.useragent.locale" to the preferred language (e.g. "en-US" or "en-GB") on the about:config page.

You can open the about:config page via the location bar, like you open a web site.
Use the Filter bar at the top of that page to quickly locate the pref.

See http://kb.mozillazine.org/about%3Aconfig

Your above posted system details show outdated plugin(s) with known security and stability risks.

• Shockwave Flash 10.0 r42

Update the Flash plugin to the latest version.

• http://www.adobe.com/software/flash/about/