I've seen these Finnish commas in English sentences plenty of times by now. There's no need to use commas for a restrictive relative clause in English.
Yes, we have lots of those (and it's my fault, by the way). We're slowly editing them out. I would argue that this particular sentence could be both restrictive and non-restrictive though. Depending on the context. :)
I thought it is Mario who is searching for the princess. Well it's a commom game theme. ;)
I typed "A boy who never speaks searches for a princess." I got a message that I missed a space, and that the correct answer is "A boy, who never speaks, is searches for a princess."
Yea, the mind of THAT quiet kid xD