North-Central Dargwa (or Dargin) language [[[Portal:Dar|dar]]]
North-Central Dargwa is in Translatewiki./net, but may need to be opened for work: Portal:Dar, and I think it is already appropriate for your demand, even if the "babelbox" incorrectly abbreviates the language name to just "Dargwa", the ISO 639-3 code [dar] is already for the north-central group of Dargwa (or Dargin).
For such dialectal division (actually you propose a dialect within North-Central Dargwa), you need at least 5-letters of extension (and the extension registered in the IANA database, such as [dar-tsuda] or [dar-cudar]) : effectively, all "extang" subtags (3-letters after the primary language subtag) are permanently reserved in BCP 47, and since the publication the reform in RFC 4646, they are deprecated by the publication of ISO 639-3, which stil considers Dargwa [dar] as a single language (but still not as a macrolanguage, because it contains no other single language).
As long as the dialect group is not in the IANA database, you would have to use a private-use extension (starting by "-x-", followed by another subtag using 1 to 8 letters or digits), such as [dar-x-tsuda] or [dar-x-cudar].
So certainly not "dar-tsud", nor "dar-cud", per BCP 47 encoding rules.
Look at: https://glottolog.org/resource/languoid/id/darg1241 (and related entries in The Linguist List), which list these dialect groups (Glottlog assigns its own private-use codes for language groups or dialects, using 4 letters followed by a number; but they are distinct from ISO 639 codes and BCP 47 tags):
- North Dargwa
- Cudaxar (or Tsudakhar) : this is probably the dialect you are speaking about...
- Nuclear North Dargwa
- Upper Mulebki
- Aqusha-Uraxi (or Uraxa-Axusha)
- Akusha (or Akkhusha, Urakha-Akhush, Urkarax)
You are refering to the North and Central group of Dargwa dialects, which is exactly what [dar] itself encodes as a single language. Your reference to "Tsudakhar" is just one possible alternate name of the language, used when focusing just on one of its dialects (and not the whole group, i.e. [dar] itself). So your request seems quite confusive.
The "South Dargwa" group is not part of [dar] and refers to two other languages with their own separate codes (in Glottolog and the linguist list, even if they still don't have an ISO 639-3 code for them). South Dargwa languages are still not encoded in ISO 639, most probably due to lack of studies and litterature about them (very few references found in Glottolog and the Lingist list, all quite recent, with only one in 2003, the next one in 2012, the first dictionary Southwestern Dargwa having been published only in 2017 by a single author for the "Sanzhi-Icari" dialect which is apparently its most common one buyt possibly partly covering some other dialects; no litterature is found for the 2nd South Dargwa language, i.e. Kajtak, also not encoded in ISO 639-3).
- It is highly probable that South Dargwa languages speakers are/were already using North-Central Dargwa (Cyrillic script) as a lingua franca or for administrative/legal purposes in Dagestan (or some other bordering languages or scripts, like Azeri with the Latin script, or Dagestan and Russian with the Cyrillic script, or Georgian with Georgian script, or the Arabic script for religious purposes), or that informal writings were developed but not widely published, using some phonetic adaptation were needed).