Which language do the French use when they search programming related information on the web?
My mother tongue is German, but when I search information on the web on topics that aren't specific to my country (such as German law or German movies) I invariably search in English, expecting to find more and better information. This is especially true for questions relating to science and engineering, where the common language is English.
The French seem to use English on the internet just as much as the German. For example, there are about as many visits to Stack Overflow per thousand inhabitants from France (19) as there are from Germany (24).
So which language will help your blog to get found?
Who is your audience?
Do you write for professional or aspiring professional programmers? They all know English.
Or do you write for beginners or amateurs, explaining basics to non-experts, introducing younger teens or older adults to software-related issues? They all prefer French.
What do you want most from your blog?
Do you want to network within the French developer community? Do you want the largest possible audience? Do you want to learn English, because you want to work for an international company, or do you want to enhance your writing skills in French, because you want to earn money as a writer for French publications?
Make a list of your blog-related goals and order them by long-term relevance. See how each goal can be better achieved by writing in English versus French.
Can you write in English?
Well, sure you can write in English, but can you write well enough that mistakes you make don't put off readers?
English might have a larger audience, but English also has a larger number of competitors, many of which write elegantly in their mother tongue. How do you feel yourself about blogs written in broken English by some Russian or Chinese blogger? Do linguistic mistakes grate on you?
How quickly do you expect to become fluent enough in your English so writing in English doesn't take much more time than writing in French?