Dee-cal is how I’ve heard it my entire life.
Youtubers love to sell custom decals to support their channels.
Some pronounce it Deck-all.
I can usually ignore different pronunciations. Deck-all grates on my nerves. How are they putting a hard k in decal?
The alternate pronunciation is in the dictionary as Canadian. It sounds so wrong.
I would say deck-al (as in Al Sharpton not as in All The Pretty Things)
I originally wrote deck-al and switched to -all.
I should have stayed with -al like Al Sharpton.
I feel the same. Yes to “DEE-cal” and a hard no to “DECK’LL.” I’m sure it’s a weird Canadian regionalism, one that’s always raised my hackles as well.
Rhymes with "fuck all’?
The etymology says its a contraction of decalcomania not that it really helps with pronunciation that much (but I don’t see where the long ee sound would come from either way? why wouldn’t it be pronounced like metal or petal rather than beetle?):
A close approximation of how I pronounce it would be more like “day-kal”
Hard no to any “dee-” variation. The word comes from a French word with an accent aigu. It should be an /e/ not an /i/.
One youtube channel that I watch is based in N Dakota. He was raised in S Dakota. Apparently deck-al crosses the border. I know the tv stations coverage crosses the border. People hear the tv announcers.
“DEE-kal” (last syllable rhymes with “pal”) is the only pronunciation I’m aware of hearing. If I’ve heard the other ones, I probably never realized what word was being said and it just didn’t register. I’m from Great Lakes dialect region US, Chicago.
Etymology of words and source languages doesn’t matter to me— pronunciations get butchered in and out of various languages, not just English.
I guess the “pecan” debate is next. 
Great Lakes dialect via Cleveland here - I pronounce it “dee-kal” usually, maybe “dee-call” if I’m feeling fancy.
Solder is another one. Europeans say it differently.
Sodd-er vs sold-er
Solder and pecan pronunciation variations doesn’t bother me.
“Deck-al” is the only pronunciation I’ve ever heard.
Tangent:
aceplace57 … isn’t the “hard k” consonant (IPA /k/) present in both variant pronunciations that you outlined?
Dee rhymes with bee 
There is a little k in cal. Like calendar.
The Webster link has 3 audio clips for pronoucing decal. The 1st and 2nd are pretty similar.
Dee-CAL is the only way I’ve ever heard it. Don’t care what the historic roots of the word are since, if I said Deck-AL to people, they’d either stare at me in confusion or laugh and call me an idiot for not knowing how to say “decal”.
Then I’d feel all self-conscious and just go with calling it a sticker.
Just because.
Just to muddy things a bit, I’ve only ever heard “DEE-cal” or “Deh-CAL”.
I have never heard “Deck-AL”, with the split between the K and the A sounds. It’s always been between the E and the C, and if it’s a short-E, the emphasis is on the second syllable.
Come to think of it, sometimes I do say it more like dih-KAL, with “KAL,” once again rhyming with “pal.” I guess when I say it it’s almost evenly stressed, but DEE-kal is my usual. But the Canadian version in that link sounds like “deckle” to me, which is a part of beef brisket.
I attended a model builder’s convention about 20 years ago. There were vendors and attendees from all over the US and many foreign countries. The only pronunciation of the word that applies to slide on transfer graphics that I heard was dee-kal.
Yep, DEE-cal for me since the 50s. I was an avid model car builder and that’s what we all called them.