Okay, I’m sure Oxford wouldn’t put out something like that but is there an equivalent for most spoken languages like Arabic, Italian, Polish or Spanish? Are they as comprehensive as one would think the OED is?
Why do you think Oxford wouldn’t put out a Spanish Dictionary?
I am certain I have seen such a work, and I have used many of their German and French dictionaries for close to a half century.
Perhaps you are enquiring about similar publishers in the countries concerned, like the Hachette French dictionaries. There is also the ossified dictionary published (I believe!) by the French Academy.
Does Oxford put out a Spanish dictionary where the definitions are in Spanish?
In regard to the OP, how about “definitive” in addition to “comprehensive”?
Yes, I would like to add the adjective “definitive” to my original post. Thank you, [color=“black”]SmackFu.[/color]
For Spanish, I believe the equivalent would be the Real Academia Española dictionary.
La Real Academia (Spanish Royal Academy) regulates Spanish language, it was created in 1714 for this purpose.
So, basically, if a word isn’t in the RAE dictionary, it doesn’t exist in Spanish language.
At least not officially 
100% correct!
The Real Academia Española was established in 1714 under King Felipe V. According to its mission, it is set up to look after the changes that may occur in the Spanish language in its constant adaptation to the needs of its speakers does not break the essential unity in all the hispanic spheres.
XicanoreX
For the Swedish language the corresponding work would be the Svenska Akademiens ordbok, or ‘the Dictionary of the Swedish Academy’. They decided to start working on it in 1786, and the first tome hit the bookshelves in 1893. By now they have published 33 volumes, and plan to be finished with the whole work in 2020.
There are similar institutions for most languages, and English is really an exception, having no centralized body officially in charge.
In Japanese, at least as far as character dictionaries are concerned Kojien is considered the ultimate reference.
In French, both Larousse and Robert are very popular, but of the two, Robert has the final word when it comes to language issues. (Larousse is more of an encyclopedic dictionary, with pretty pictures and all.)