The Norwegian sentence doesn't convey this meaning here, it's just stating that something happened a long time ago.
I think "Det er lenge siden sist" would be a better translation for "It's been a long time". But then again, you might just say "Takk for sist" instead if you were meeting someone you hadn't seen in a long time.
"Det er lenge siden vi har snakket" = "It's a long time since we've talked".
It's closer to:
"Aren't you the famous football player" = "Er ikke du den berømte fotballspilleren?"
"That is a long time ago" = "Det er lenge siden."
Some people might write "Dét er lenge siden" to distinguish between what is the difference between 'it' and 'that' in English. This is still not accepted in Bokmål, but may be in the future.
It doesn't sound correct to me without the indefinite article. To my surprise Google searches show that it is frequently used in informal writing although it doesn't occur in books except very occasionally in quoted speech. It's certainly not something you would hear in British English. It may be that it occurs in US English, and with some accents the "a" can be indistinct. Possibly this is intruding into current informal writing. Perhaps an American could chime in and enlighten us.
There is of course the Christmas carol "Mary's Boy Child" made famous by Harry Belafonte. It was composed by Jester Hairston (1901-2000), the actor, preserver of ❤❤❤❤❤ spirituals, and all round top person. Hairston wrote the song in a calypso rhythm and using a West Indian dialect. Presumably for commercial reasons, and in my view most unfortunately, the dialect was toned down in the Belafonte version. In the original version the first verse was:
Long time ago in Bethlehem,
So de Holy Bible say,
Mary boy chile Jesus Christ
He born on Chrissamas day.
Note in particular the spondaic "Christmas" was originally a dactylic "Chrissamas" which fits much better into a calypso rhythm. I would argue that the lack of an indefinite article in "long time ago" here is a deliberate choice by Hairston starting the song with a clear indication that it is not in standard English.
There is some grainy video here of the great man rehearsing Danish singers in the song.
I think that's quite different. Starting a sentence with "Long time ago..." is like starting with "Dude told me that...". It's quite normally done in questions like "You been there yet?" where "have" has been omitted. This is very unlike omitting words from the middle of sentences, where it just sounds foreign.
American English speaker here. The sentence "that is long time ago" is completely wrong to my ear. We would say, "that was a long time ago", or "it's been a long time (since then)". It's not slang, its just sloppy, though we would understand what you mean. You'd get odd looks, and possibly be corrected. I have heard people say things like, "that's a long time ago", but it's very informal and not grammatically correct.
I agree that past tense is more usual and sounds more natural, but it does depend on context. To take the example of the football player above, if he said he was famous in the 1970s it would be perfectly acceptable (at least in British English) to say 'that is a long time ago'.