Nothing too huge, just things you might notice after awhile if you're really paying attention and don't shrug it off as a slight speech impediment -- which it kinda is, I guess, but y'know what I mean, right?
They have a "flat" face like humans and have obligate (hyper)carnivore teeth, if that makes any difference.
Edit:// They do have lips, by the way.
You don't need to be an alien to be unable to articulate certain sounds in foreign languages.
I remember when I started studying English in school that the pronunciation of "the" or "two" was a nightmare, with many classmates pronouncing "two" as "chew". Then moving on in life I have heard Germans pronouncing "one" as "won", or Japanese boys asking me if I "riked" sushi, with some mispronunciation even making it to become the brand of a product
“now zat iz what you call eh phet bast-ard” He had used the expression Fat bastard often to describe things that were great but hearing it in a French accent made it so much funnier.
Shibboleths are pretty common in several languages
Shibboleths have been used throughout history in many societies as passwords, simple ways of self-identification, signaling loyalty and affinity, maintaining traditional segregation, or protecting from real or perceived threats.
What makes real wonder is how can an alien fully articulate human sounds with just a few problems, and I underline the few.
It depends on the exact physiology you give them and how that differs from humans. For example, suppose they can't flatten their tongue the way humans can so it can't take up more lateral space in the mouth, which means they can't close the gap between tongue and teeth enough to make the lateral fricatives (not present in English, but is other languages).
Similarly, for a real easy one, if they have sufficient enlarged canine teeth, they might not be able to bring the lip into contact with the teeth on the other side of the mouth, which means they can't produce the labiodental fricatives (/f/ and /v/, for example), which is going to stand out in a lot of languages, including English. What will happen in that case is that they'll end up sounding like a ventriloquist: when they're speaking rapidly, the listener might not realize they're not pronouncing those sounds because most of the time context will clue the listener in on what word is being used so they'll mentally fill in the blanks and perhaps not even notice it, but if you're listening for it, it will stand out.
As an example, the parts of the preceding sentence might sound like "...they'll end up sounding like a wentriloquist... most o' the time... but i' you're listening".
Or, to use a more pop-culture reference, they could be wandering around asking where they can find some nuclear wessels.
Fangspeak?
https://tvtropes.org/pmwiki/pmwiki.php/Main/FangThpeak
A speech impediment that strikes vampires (and other fanged creatures, as well), where their spoken s's become th's or, more often, sh's.
Here's the technical explanation for why it happens: False teeth tend to cause sibilants (s; soft c, like the first c in "circus"; and sometimes z) to be mispronounced because the prosthetics force a change in the position of the tongue. Interdentals (th) are made by the tongue going against the upper incisors - sibilants are more likely to be mispronounced as interdentals when the incisors are altered or missing, as with fake buck teeth or missing front teethnote More common with prosthetic fangs are for the sibilants to become post-alveolar fricatives (sh), caused by the tongue being forced back by the wider and longer canines.
The article describes workarounds by actors who are wearing fangs and writers who don't want their badass vampires lisping like Sylvester the Cat. I say your hypercarnivore aliens just go full on slobbering Sylvester. They look like him too, and they crave Tweety Bird.
A few might look like Daffy, to keep things fresh. One looks and sounds like Snagglepuss but that one filed down its teeth.
if the aliens are physiologically similar to humans, it could be as simple as adjusting the larynx. An example is Neanderthal's had a shorter larynx and thus would have had a smaller range of sounds they can create and thus would have had a stunted ability to communicate.
They would have had a real difficult to communicate with Homo sapiens and not able to produce all the sounds they used. It is also assumed that Neanderthal's had a squeaky, high pitched voice. They looked almost human, but we would think they sounded weird when they talked.
Your alien needs to be able to invoke a gas flow (easiest: breath out)
Speech sounds propagate through air. If you want to make yourself audible, breathing out air is part of that, in Earth's atmosphere, resonances and friction sounds will propagate and reach the listener. The alien should understand that basic concept, else it won't be able to learn speech by mimicing it. Once it grasps the idea, mimicing vowels will be no problem, as long as the alien has some cavities it can resonate while breathing out.
Some consonants will be a challenge, though,
The teeth may spoil some consonants
Unless the alien has giant and flexible lips over its "hyper" carnivore teeth, any "v", "w", or "m" or "p" or "b" subtleties would get lost, teeth are in the way, adding a slushing sound to everything, the alien has no lips, or its lips cannot be closed. Nasals sounds could be feasible, depending on the shape of the head and nose. Without a nose, the alien just needs some smaller, second cavity to mimic "n" or "ng".
Tongue and soft tissue
Shape may be no hurdle at all. The alien could mimic sounds using different parts of the mouth we do, its physiology would be different, sound similar. A flexible tongue would come in handy to do these tricks !
For realizing "g" and "z" and the difference between them, the alien must have certain soft tissue in the mouth. Without soft tissue, I doubt if the alien can produce credible voiced consonants, like "l" and "j". Also, plosives "d", "t" and "k" depend on softness differences, without soft tissue, it would be difficult to produce them, or they all sound like "k".
Hopeless ? No..
It does not have to be perfect
We can understand Donald Duck. Or a parrot. Humans are perfectly capable of interpreting speech, also when the speaker is impaired in some way. As long as the error made is consistent, the brain can recover skipped sounds, disambiguate similar sounds, or cope with a different vowel.. even wrong syllable timing can be comprehensible.. after a few minutes, a human listener can get used to that and regard the errors as "alien accent".
