Is it considered to be plagiarism when a researcher translates a little-known paper from another language and uses this text, without citing reference, as his own? I understand that to use results of someone’s research work is ethically improper but does it carry a real punishment?
Short answer: Yes, it is plagiarism.
Slightly longer answer: In the process of translating a little known text and using it nothing has changed about the fact, that you copied text from someone else without citing it correctly. I would consider it unethical. Why would you not want to cite it anyway? You got the source, you can cite it! You shouldn't bother about possible consequences, because doing it right in the first place will prevent you from getting into trouble.
In many legislations (e.g. Germany) it is a violation of copyright as well: the translation is considered a derivative work/adaption. This means you need the permission of the copyright holder (typically publisher or author) to publish your translation. Citation alone is not sufficient.
As for "using it as your own": Your translation may anyways qualify as an own work, depending on how much original work you put into the translation. (Computerized translation does not qualify.) A good translation is not a mechanical process but constitutes a creative work in itself and therefore is considered a work with its own rights (at least in German copyright law).
Again, this doesn't change the fact that you have to obtain permission of the copyright holder of the original work for publishing your translation. Doing this will typically take care of possible issues with plagiarism as well: re-use authorizations tend to specify how to reference the original work (slight disclaimer: the re-use authorizations I've seen so far were not for translations but for graphics and tables).
Yes. Plagiarism is, at its core, using someone else's work and claiming it as yours. The fact that you translated it does not diminish the fact that you are using someone else's ideas.
I should add that there is of course value in itself in translating a previous paper, and you will get some credit for this even if you make it clear that it was someone else's ideas that you are translating.