Casting spells with verbal components is chanting of esoteric words recognized by spellcasters and is otherwise language free.

Think gibberish vocalize

Quote from Larix_Shady >>

I understand all that - but my question was:  if you can cast a spell in different ways with different languages - how would a human PC who does not know a language, know that a fiend is casting a spell versus just talking.  So its not a question of flavor, its a question of who/what they are encountering.

i mean, the real answer here is "because the mechanics of the game require it"

If you need an in-game explanation, just write it off to the PC getting a sense that the string of otherwise unintelligible words are in some way impacting the Weave, and thus must be a spell

Are you actually asking a question, or just looking to legitimize your headcanon? Regarding exactly how the phonemes work, I don’t believe there’s a single official answer, particularly not in 5e specifically. That said, I’m fairly sure it doesn’t follow the Harry Potter model of everyone using the same pseudo Latin phrase for a given spell. In terms of how a spell is recognized, the official position back when they released Xanathar’s was that to recognize a particular spell you had to either spend a reaction as it was being cast or an action after it was cast to make an Arcana check. Naturally, almost no one uses this suggestion since it largely defeats the point of counterspell if you can’t know what you’re countering, and as a video I watched noted it can create this adversarial dynamic between the DM and casters whenever either side is casting since it makes people want to “hide” their spell until after they’re told if it was countered or not. It does indicate that components are not so standardized as to be immediately knowable, though.

If the vocal component is a set of specific sounds not part of a language then yes you might be able to recognize the verbal component and be able to counterspell it. If it is a spell you don’t know then it would be far more difficult. The further problems are spell like abilities - including things you can cast without using a spell slot - how to deal with that? It is also always possible the some monsters, aberrations and possibly demons especially, could have found alternate sets of sounds that also work and so you might not know just from the sounds. To my mind it’s not really that much of a deal as counterspelling is - to my mind- mostly a trap. In most circumstances you are going to be better off doing something other than counterspelling. But, yes, there are times when you really need it - like when your AoE that should have taken the bbeg and major minions out doesn’t, so now they can nuke your badly injured party with their own fireball for a TPK unless you counterspell it. 

To a trained ear magic sounds magical. A chef on vacation might not know the local language, but they know if they smell something cooking. A wizard hears another wizard wizarding and even though the words aren't familiar, they can feel the energy being summoned and hear a bit of something extra in the voice that puts up a "magic going on over here" red flag.

Quote from Larix_Shady >>

I understand all that - but my question was:  if you can cast a spell in different ways with different languages - how would a human PC who does not know a language, know that a fiend is casting a spell versus just talking.  So its not a question of flavor, its a question of who/what they are encountering.

 
Ok, sorry for responding pre-coffee, and not understanding your question.
I will do it again, i always do.

To try to actually answer your question: There are multiple ways to rationalize it. And you could even have them all depending on the character's and how they do it.

1:The Arcana skill lets you understand the principles of magic and its manipulation. While highly abstracted on how it works, it would give you an understanding of what someone manipulating the weave is doing. Say someone is casting in an unknown language. Do you need to know the language to hear the sounds they make? No. Even someone who doesn't speak spanish will hear and recognize a rolled r, a non Danish speaker will be able to hear glottal stops in their words, even if they have no idea what it is called. You are basically seeing and hearing things that manipulate the weave, and if skilled, you know what those things do. 
" Ok, they made the drawing down hand motion, and i heard 'Brzzt' 'Dothzh' and 'Arcan' so that means they are likely doing an arcane lightning spell in a line. Lightning-Bolt most likely."
This one is on brand for a wizard, artificer or Cleric.

2: Another option is to say that you can see or feel how the weave is being plucked at, and that tells you what they are doing. It is akin to watching someone pull their arm back, and knowing they are about to punch, or smelling the moisture in the air and knowing it might rain.
A Sorcerer with their innate magic in their blood, or a Druid who feels the flow of nature and its energies would be on brand for this one.