Tim Curry was really in the English dub of Sammy: A Turtle's Tale!

Hi, Alex Hartsell. For your information, you ought not make a habit of citing a Wikipedia article as an evidentiary source, and to some extent, this notion applies to all encyclopedias, depending upon what is being argued. I'm not sure about the Amazon website, but we have to be mindful that it too has occasional errors, often derived from IMDb, a crowd-sourced repository of some public information.

Neither Wikipedia, Behind the Voice Actors, nor the information on Amazon are proof that someone is credited in a production. As with IMDb, anyone with an account can add information to Wikipedia or Behind the Voice Actors, and the information on Amazon can be incomplete, mixed up with another title or just plain wrong.

The only truly authoritative source for IMDb is the full on-screen ending credits of the production. Therefore, the best way of proving that someone is included in the credits is to take a still of where their name appears, upload this (for example, to Google Drive) and make it viewable to the public, and include a URL to the image in your explanation of the credit submission. I have tried linking to full shorts online as evidence, but I've found that tends not to be accepted (as, I suppose, the staff aren't going to look through the credits themselves to find the relevant part); it's better to provide a still of the specific part of the credits.

Linking to other places in which the person is credited officially – such as in trailers, on video packaging, and on official websites – in addition can help, but none of those are the main determining factor.

Out of the links you have given, the only semi-authoritative source which I can access in the UK is the UK trailer at https://www.youtube.com/watch?v=FFu9CXyQQ0Y

That trailer, as is the convention, has a list of the lead voice cast towards the end of it:

Tim Curry is nowhere in this list. Though the trailer does not list the full voice cast, it is highly improbable that someone as well-known as him would be cast but then not mentioned in the trailer, which does credit some relatively less well-known British actors. So this evidence you have provided actually contracts your claim, not confirms it.

I can make an informed guess about what has led to this confusion, however. While the information on Wikipedia, Behind the Voice Actors and so on cannot be used as authoritative evidence, they can be useful as starting points for finding out the sources behind what they claim.

https://www.behindthevoiceactors.com/movies/A-Turtles-Tale-Sammys-Adventures/Fluffy/ credits two different English-speaking actors with the voice of Fluffy: Kayvan Novak (who is credited in the UK trailer) and Tim Curry, who is not.

It's possible, therefore, that this movie has had two different English-language versions. It is not unheard of for animated movies and TV programmes (particularly ones aimed at young children) and video games to be released in the UK and Ireland and in the USA and Canada with different English-language versions, either ones with completely different voice casts or ones in which some voices are the same and some are different.

Movies and some TV series (particularly Japanese animated ones) can also sometimes get more than one different English-language dub from the same country but at different times. (IMDb has no rules in its submission guides about how to deal with these cases, but that needs a thread to itself to enquire about it.)

I suspect that one of those situations might be what has happened here.

After doing some digging around, I found that the film's page on Flanders Image (an official database of Belgian productions, which makes it at relatively authoritative, though it does not essentially contain the same information as the on-screen ending credits) lists a completely different lead cast, which does include Tim Curry and not Kayvan Novak, on both the page itself and a poster embedded on it.

It also appears that, from that cast list being on the poster on Flanders Image and the same poster being on the animation studio's site, that the cast including Tim Curry is the one that the mouth movements are animated to, while the Dutch, French and German versions that were released in Belguim and the UK-released version with Kayvan Novak are all ineligable for being in the cast list, following IMDb's current rules for voice credits. (It is quite common for European animated features to be animated to an English-language version, even when they are produced in countries in which English is not one of the primary languages spoken, then dubbed into their own language.)

The casts of all other versions of the film should instead have their credits on it mentioned as "other works" in their biographies.

So, in the end, I think you are right, but you need to provide the correct evidence with your submission. Wikipedia and Behind the Voice Actors do not count, and the UK trailer actually contradicts what you want to submit. What would be best, I'd guess, is to link to both a still of the voice cast in the ending credits (to prove this person is mentioned in the full ending credits, with which character name and in which order) and the film's page on Flanders Image (to prove that this voice cast is the one that the mouth movements are animated to). And you will need to explain what each link is proving.

I believe I have this movie somewhere, but if I recall, UK and US versions indeed feature different voice actors.