The 3rd scenario of Voice of Isengard: Into Fargorn:

When time keyword of quest card is trigered and it says that Mugash return to encounter deck, i think you don't shuffle that deck. It doesn't say nothing about shuffling, so i don't shuffle. Is it correct?

The quest cards say "Shuffle Mugash into the encounter deck"

in spanish text says (translation to english ): ...add Mugash to the encounter deck . Nothing about 'shuffle'

I guess it is safe to assume that the English version is the authoritative version.

in spanish text says (translation to english ): ...add Mugash to the encounter deck . Nothing about 'shuffle'

Hmmm this is really odd.... I suggest to play English card then. Practice a language and no any confusion.....

"Add Mugash to the encounter deck" is unclear anyway - add him where? on the top? the bottom? wherever you like?

I guess to the top, because he is guarded, so next round, it becomes guarded card