Learn English – Word for someone who can’t keep up with the times

Your question already contains the best adjective: nocturnal refers to being active at night and sleeping during the day. I don't know about elegant, but this is certainly the most technically-correct word you're going to find. Unfortunately, it doesn't have a noun form, but that can usually be fixed by rearranging your sentences a bit.

Sally was a night owl who was always late to early-morning meetings.

Sally's nocturnal tendencies made her chronically late to early-morning meetings.

I do want to note that in most contexts, I wouldn't interpret either night owl or nocturnal (when applied to people) as explicitly sleeping during the day and working at night. Both words just imply that, left to her own devices, Sally would prefer to go to bed late and get up late, where "late" could have different definitions for different people. If you need to make it clear that someone sleeps during the day, then either you need to say that ("sleeps during the day") or refer to them working the graveyard shift (which admittedly is not the most elegant of phrases).

First thing I would like to point out - chugalkhor doesn't seem to just mean 'can't keep a secret'. More like somebody who is actively complaining about / revealing others' wrongdoings, not 'not keeping secrets'. So the question's premise itself seems wrong.

Now going on to an English word:

An English word meaning the same thing as chugalkhor:

• tattletale: someone who gossips indiscreetly
• tattler: synonym of tattletale
• taleteller: synonym of tattletale
• telltale: synonym of tattletale
• sneak: British Informal tattletale; informer
• snitch: Also called snitcher; an informer.
• backbiter: one who speaks unfavorably or slanderously of a person who is not present.

-Source of words from shabdkosh.com and source of meanings is from dictionary.com

Of the above words, tattletale and the other highly similar synonyms related to tattle come closest to the way your given word is used.

(eta) chugali is entirely defined as tattling and telling tales in this Hindi-English dictionary I just discovered.

A sneak is regional and also implies cowardliness, and a snitch can often specifically imply one informing the authorities such as the police. And a backbiter can backbite in more ways than tattling - he can just spread uncomplimentary information (true or false) unrelated to actual wrongdoing.

An English word for someone who can't keep a secret

• blabbermouth: a person who talks too much, especially indiscreetly.

• indiscreet: not discreet, i.e. not "judicious in one's conduct or speech, especially with regard to respecting privacy or maintaining silence about something of a delicate nature."

A blabbermouth is not exact, since a blabbermouth can talk too much about things that aren't secrets, but the phrase usually does imply a person who wouldn't be able to 'keep it in' while blabbering if they were in possession of a secret.

But it's a bit closer than a plain indiscreet which can and does also apply as frequently to conduct not just talk, and more importantly can refer to being careless with one's own affairs and secrets, not just others'.

Loose-lipped is another colloquialism for the concept, if not one you're likely to find in a dictionary. One of the sayings to popularize the phrase is the WWII slogan Loose lips sink ships; but apparently the phrase was also used by Virginia Woolf (1882-1941).