I haven't read Das Kapital, and even then I wouldn't be likely to understand much of it, especially as it wasn't written in English in the first place. Also, it would be tantamount to learning about vacuum-tube electronics, which has become mostly obsolete by now. Great minds can come up with right ideas but also with wrong ones. Galileo and Sigmund Freud were wrong on a number of things as well.
First of all, there are translations of Das Kapital
. Considering that capitalism is built on relatively old theories you might as well call capitalism obsolete. Freud may have been wrong on some things, but he was a nobody to the U.S until someone recognized his potential as a marketing tool. His psychology is the basis for the, "You MUST buy this!" advertising. And also the, "This'll make you look sexy and trendy!". Considering both techniques are still employed today I'd say they're far from obsolete, I'd even say that Freud in this case was ahead of the curve.
As for Galileo, I'm not sure what he has to do with anything. If you're implying that the Earth revolving around the sun is obsolete, then I'm not quite sure what else to say to you.
Galileo wasn't wrong about that; it's since been proven a number of times. But other lesser known ideas of his have been. From a scientific standpoint, Freud was wrong about some things as well.
There are translations of a lot of things, but some are better than others. Take the first line of the Franz Kafka novel The Metamorphisis
. Many translations say that Gregor Samsa woke up to have turned into a giant insect. The actualk sentence was that he transformed into a giant vermin
. He wasn't an insect; insects only have six legs and he had many.