Al Jazeera is attempting to make itself into a more mainstream news agency, e.g., CNN, BBC, etc.
Reporting in English broadens their market.
http://english.aljazeera.net/NR/exeres/55ABE840-AC30-41D2-BDC9-06BBE2A36665.htm
Al Jazeera has long been a respected and "mainstream" (whatever that means) news agency. In fact they have won numerous awards for their reporting. Here are a few taken from Wikipedia:
In December 1999, Ibn Rushd (Averoes) Fund for Freedom of Thought in Berlin awarded the "Ibn Rushd Award" for media and journalism for the year to Al Jazeera.[62]
In March 2003, Al Jazeera was awarded by Index on Censorship for its "courage in circumventing censorship and contributing to the free exchange of information in the Arab world."[63]
In April 2004, Webby Awards nominated Al Jazeera as one of the five best news Web sites, along with BBC News, National Geographic, RocketNews and The Smoking Gun. According to Tifanny Schlain, the founder of the Webby Awards, this caused a controversy as [other media organisations] "felt it was a risk-taking site".[64]
As for their english site; In March 2003, they launched their first english website offering coverage on the Iraqi conflict (or what would become so). The site disappeared shortly there after and the government kicked their smear campaign of Al Jazeera into high gear... draw the conclusions you will.
Link about first english website (not the best source, but all I could find. Note: this is not the National Inquirer just so you know):
http://www.theinquirer.net/?article=8509