In a recent question there was some confusion as to whether or not Thieves' Cant is a language, a class feature, or both.
Which is it?
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It's a class feature (though I'm not sure anyone's debating this) because it's detailed in the list of a Rogue's class features. Beyond that...
I think you've hit one of those places where the game rules have attempted to fit a round peg into a square hole. Mechanically the way the rules treat thieves' cant (as a language) appears to jar slightly with the way you'd most likely rationalise it being used narratively.
It's referred to explicitly as a langauge in a couple of places:
With your DM’s permission, you can instead choose a language from the Exotic Languages table or a secret language, such as thieves’ cant or the tongue of druids. Basic rules
You might invent additional secret languages, besides Druidic and thieves’ cant, that allow members of certain organizations or political affiliations to communicate. DMG
It's worth noting that both of these quotes refer to it as a 'secret language and that it is not listed in either the standard or the exotic language lists. This is support for it sitting slightly uncertainly alongside other normal languages.
It's a written language:
You understand a set of secret signs and symbols used to convey short, simple messages, such as whether an area is dangerous or the territory of a thieves’ guild, whether loot is nearby, or whether the people in an area are easy marks or will provide a safe house for thieves on the run. Basic rules
So, these symbols are portrayed as an international code known to thieves - not in any way localised.
But it's not a spoken language:
During your rogue training you learned thieves’ cant, a secret mix of dialect, jargon, and code that allows you to hide messages in seemingly normal conversation. Only another creature that knows thieves’ cant understands such messages. Basic rules
The fact that thieves' cant allows you to hide these messages 'in a seemingly normal conversation' means that it isn't itself an alternative spoken language.
Proficiency in thieves' cant allows a thief to have an apparently innocuous conversation in Common (or any other spoken language) while employing a series of techniques in order to communicate a specific message, entirely unbeknownst to unenlightened listeners. If they were speaking either another language entirely, a variant of Pig Latin, or even something like cockney rhyming slang, then it would not appear to be 'a seemingly normal conversation' to a casual observer - even if they'd be none the wiser to the words actual significance.
Well, yes and no.
Yes - mechanically the game treats it as one, referring to it within lists of languages on a couple of occasions - you can even take it as a language option if your DM allows. It also has elements of a basic written language, as it's able to convey 'short simple messages' through signs and symbols. And understanding of these symbols is not apparently in any way localised - unlike a real world cant or argot.
However, No, it's not a language in the plainest sense. It cannot be spoken on its own but relies on the thief speaking another language while applying a series of verbal techniques in order to disguise their true meaning. Thieves' cant can apparently be applied to a conversation in Gnomish or Abyssal just as easily as one in Common and it therefore must be considered more of an array of obfuscating oratory techniques than any kind of linguistic dialect.
During your rogue training you learned thieves’ cant, a secret mix of dialect, jargon, and code that allows you to hide messages in seemingly normal conversation. Only another creature that knows thieves’ cant understands such messages. It takes four times longer to convey such a message than it does to speak the same idea plainly.
In addition, you understand a set of secret signs and symbols used to convey short, simple messages, such as whether an area is dangerous or the territory of a thieves’ guild, whether loot is nearby, or whether the people in an area are easy marks or will provide a safe house for thieves on the run.
Linguistically, Thieves' Cant is a cant (unsurprisingly) or argot. Its use in D&D is based on the real-life thieves' cant used in Britain originating in the 14th century and popularised in Elizabethan literature.