This came up in a game recently, while a player is wearing this, do they 'know' what language is being spoken, or does the spell only grant understanding (ie everything it hears is translated into common)

That's a great question.  I personally believe it should be up to the GMs discretion, but, I'd lean towards the PC not knowing what language it is if you want to keep it fair and balanced and all

Thank you for replying.  In one instance, I'm the player and my DM ruled that it just comes to my character in common without knowledge of origin.  I've DMed for another player where I described that the helm streams into your consciousness the translation of Orcish, Elvish, etc.

I was primarily wondering if there was an "official" answer, or errata.  As there are situations where it's nice that the player doesn't know, and other situations where it's nice that the player DOES know.

Does this item require that the player has the ability to cast spells? My barbarian has one and he cannot cast spells.

You can't cast it while raging but he can cast it when not raging.

Thanks for confirming. =)

I would rule that since the helm grants you the ability to cast the spell. Whatever the spell states is what I would go with. 

The spell doesn't tell you the language you're translating.

However, the "touching" the text you're looking may allow for "knowing" the language you're reading.

Maybe they thought that if it was called the Helm of Understanding it could be mistaken for something that gives a bonus to insight. Also just being helmet of the spell it casts makes things easier.

So assuming the 4x prices for Barding, buying one for a Large mount would cost at least 404-to-2000 gp? Finally, a way to converse with my awakened Riding Horse who only knows Sylvan!

Based on the wording of the spell it lets you cast, you "understand the literal meaning of any language that you hear" implying the language is being spoken as is, your character just understands the meaning of the words. The way I see it, it's akin to reading Subtitles while listening to anime. As you do it, it's like you're not really hearing the language anymore, but your own mental reading of the subtitles.....right up till you turn away and then go "Wait, the **** did they just say?"

But I digress, that's the best way I can describe it, it gives your mental subtitles more or less....hell if the DM wanted to be cheeky they could rules the spell literally adds subtitles to their speech from your perspective.

I don't speak german or elvish but I know when I'm hearing them. but I guess if it translated it so perfectly and instantly, you might not realize that another language is being spoken? like how harry didnt realize he was speaking parseltongue at first?

In my game, one of the players conceived this as a "helm of subtitles" that shows subtitles whenever anyone is speaking in a foreign language. :D

indeed. 

the wearer of the helmet understands the language.

but if another party member is listening as well, that person can hear what langauge is spoken - while you get the insta translator.

another fun thing according to the spell text - "you understand the literal meaning of a language spoken".. so if the speaker is talking in slang, you dont understand the meaning but only the direct translation?

- "this soup is the dogs bollocks"

- "what did you say the soup was made of??" o.O

If it were my table I'd say you could because the magic comes from an item not you, just like being able to rage and use a wand of magic missile if you want, it's not you physically casting the spell or requiring concentration 

I ruled that you still hear or see the language as is, in addition to the translation.  Because of this, you can make an Intelligence (History) check to see if you recognize enough of the sounds to make an educated guess about what you're hearing.  Depending on the language, Arcana (Draconic, Elvish), Nature (Primordial, Sylvan), or Religion (Abyssal, Celestial, Infernal) could work too.  If it makes you feel uneasy just to read or listen to, it might be Daelkyr, AKA Deep Speech.

translation.

Total noob here running a solo campaign, but need this item for the story to progress. It would seem that it's basically a universal translator. 

Correct, since it is at will and there is no concentration. It just lasts for an hour. And in 5e helms don't give you AC bonus or anything so you can style/flavor it any way you want.  Tryin to get my DM to let me create this as a purple headband for my Tortle (Knowledge Cleric/Scribe Wiz).

In my campaign, I use the classic Sci Fi translator microbe rule. You just hear it as your most familiar language. No way of knowing what language aside form common sense or deductive reasoning.