No. tl;dr: Small benefit, high cost.
As a tool to learn language
One of the answers said something along the lines of "and it helps me to improve my German". Actually, programming taught me a great deal about English language and international communications. Personally, I taught programming myself during high school ("Gymnasium") and jumped right into the English world at the very start.
Syntax is English
Moreover, since most languages are international and use English commands, a command of the English language is suggested anyways: How are you going to remember what functions such as implode() do, if you do not understand the word? Point taken, it takes more knowledge than just keywords to formulate questions and answers in English - but English is a mandatory language from the 5th grade on, and the trend is still improving for younger generations.
Redundancy and Scale
Many of the complex questions are already answered in English. There are great scale effects to a site like stack overflow, which are that you can both search for questions that many people have asked and also that your answers will potentially serve many others who have the same problem later on. Narrowing down on a specific language takes away these externalities.
You don't need excellent command of the English language anyways
As long as you can formulate questions and answers in basic English, you're fine. A lot of people actually edit questions in stack overflow, improve phrasing and similar. That way, you actually also learn something about English.
But what if I really don't understand something in English?
Due to the clear structure of stack exchange, it works great with translators such as google translate - which provides excellent translations between German and English.