"He feels a failure", though not technically incorrect, sounds very, very unidiomatic.
"Feels a failure" is about half as common on Google as "feels like a failure". It appears that the former mostly is used on UK and Australian sites, and the latter in the US. As an Australian, both forms sound OK to me. I'll report it.
Fair enough! I've gone back and added it, now that I have access to those tools. :)
With Wikipedia as a source, the best I can find is "Bluffsyndrom", where the word "Bluff" is used both as the "cheat" in poker and other card games, but also when you portray yourself to be someone else.
It's not grammatically incorrect, and the meaning makes sense. It's also accepted. :) But it's not really very idiomatic.
This could be rendered "He feels himself a failure" which is pretty idiomatic but a tad more formal than "He feels like a failure;" or could be "He feels himself to be a failure," which is correct but slightly stilted, a flaw shared by the terse but also correct "He feels a failure."
In my SV<->DE dictionary misslyckad was also translated into missraten -> wayward. Is this an acceptable translation?
No, it only corresponds to the "failed" or "unsuccessful" sense of the adjective missraten.
As a Scot I have to translate what I'm thinking into English/US English before submitting anything into a Duolingo answer, so He considers himself a failure may be a bit pompous but surely it isn't wrong?
It's a perfectly fine phrase, of course, but it corresponds better to a different one in Swedish - for instance, Han ser på sig själv som ett misslyckande. Note that one sentence uses an adjective, and another a noun. We do not generally allow for translations that change too much into something else, and though I absolutely get your argument, this is a very good example of that. Obviously, in real life, your translation might have worked great.
Also, I'm not a native English speaker, but wouldn't you say that feeling like a failure is a little more temporary, while considering oneself a failure is more like a permanent state?
Det är en svår fråga för mig: jag känner mig misslyckad värje dag på Duolingo svenska! Ja, I tänker att du har rätt. Men minns du Jack Wild i 'Oliver!'?: "Consider yourself at 'ome, consider yourself one of the family". Tack, jag kommer att få min rok.