In English - and no industrial films named by committees
Well, since this isn’t GQ I suppose we can throw out guesses?
Offhand, “The Englishman who Went Up a Hill and Came Down a Mountain” seems like a good candidate.
“Dr. Strangelove” is also a candidate if you include the “or” phrase that went with it.
Don’t Be a Menace to South Central While Drinking Your Juice in the Hood
It seems quite long and it is the first title that popped into my head.
The Persecution and Assassination of Jean-Paul Marat as Performed by the Inmates of the Asylum of Charenton Under the Direction of the Marquis de Sade (1966) (UK: complete title)
The Incredibly Strange Creatures That Stopped Living and Became Zombies. An MST3K episode.
This isn’t as long as don’t ask’s contribution. But The Talented Mr. Ripley’s full name is The Mysterious Yearning Secretive Sad Lonely Troubled Confused Loving Musical Gifted Intelligent Beautiful Tender Sensitive Haunted Passionate Talented Mr. Ripley. IIRC, Each adjective flashes on the screen briefly during the opening title sequence.
I thought it was The Incredibly Strange Creatures That Stopped Living and Became Mixed-Up Zombies. And that makes all the difference.
I Killed My Lesbian Wife, Hung Her on a Meat Hook, and Now I Have a Three-Picture Deal at Disney (1993)
These all sound like fascinating works of art. Especially that last one. 
You want a work of art? How about Oh Dad, Poor Dad, Mama’s Hung You in the Closet and I’m Feeling So Sad?
anything by lina wertmuller:
Cafeteria, or How Are You Going to Keep Her Down on the Farm after She’s Seen Paris Twice
Runtime: one minute.
Damn. And I call myself an MST3K fan.
Not the longest, but one of the strangest,
Can Hieronymus Merkin Ever Forget Mercy Humppe and Find True Happiness?
Anthony Newley, fresh from the success of “Stop the World; I Want To Get Off” was greenlighted into this wretched indulgence of a movie which is so bad it has a horrid fascination including:
- A surprising amount of skin for 1969
- Georgie Jessel (!) as either God or Death
- Milton Berle as Goodtime Eddie Filth (The Devil)
- A young Joan Collins as Polyester Poontang
- Stubby Kaye as a stagehand
It’s all explained (as well as possible) with stills from the film at this site.
Don’t say you weren’t warned.
For modern films, how about Woody Allen’s Everything you always wanted to know about sex
but were afraid to ask.
Dr. Strangelove, Or: How I Learned To Stop Worrying And Love The Bomb.
One of my favorite films, I’ll use any excuse to promote it 
.
According to Guinness, it’s this film, but they have a longer title than what the IMDB says:
Fatto di sangue fra due uomini per causa di una vedova - si sospettano moventi politici. Amore - Morte - Shimmy. Lugano belle. Tarantelle. Tarallucci e vino.
Second place is Marat/Sade, courtesy of don’t ask.
Next is Les yeux ne veulent pas en tout temps se fermer ou peut-être qu’un jour Rome se permettra de choisir à son tour, listed as an alternate title in the IMDB to this.
How about It’s a Mad, Mad, Mad, Mad, Mad, Mad World? (I’m not too clear on how many "Mad"s there are.)
Just because those words flash on the screen during the title sequence does not mean that they are part of the title. All titles are registered with the MMPA. (and they must be approved by the MMPA) The registered title of the film is The Talented Mr. Ripely.