As a medic main in the day, this is exactly what came to mind. I always thought it meant "let's go, let's go", but him saying "quickly, quickly" is interesting. All the American character's give commands on what to do (e.g. "let's go!", "Move 'em out", etc) while Medic is yelling how you should move. Is this just a coincidence, or indicative of the mannerisms of native German speakers?
We don't usually use "rapid" with animals.
You could talk about a rapid development, but animals and people are usually "fast" or "quick".
(The adverb "rapidly" is used a bit more widely; you can "run rapidly", for example, though "fast" or "quickly" would probably still be more common.)
Well, probably the best translation here is "The animal is fast" , so considering "das" as definite article for the neutral name. For the translation of "This animal is fast" it is better to say "Dieses Tier ist schnell" or probably also "Dies Tier ist schnell"; in fact I remember I read in another comment that "dies" can be used regardless of gender. Please a native German correct me if I am wrong.
"Dies Tier ist schnell" looks odd to me. You might hear that, but I think that's just a short version of "dieses" - and might actually be pronounces slightly differently, almost as if it's "die-s" with the "s" as a separate syllable compared to "dies" (for neuter) which is just one syllable.
And "dies Frau" is just wrong.
What you might have heard is that you can use "dies" as a demonstrative pronoun for any gender, but not as a demonstrative adjective.
So you can say:
Dies ist ein Mann.
Dies ist eine Frau.
Dies ist ein Kind.
But you can't say all of:
xx Dies Mann ist groß.
xx Dies Frau ist groß.
maybe? Dies Kind ist groß -- better, "Dieses Kind ist groß".
Not in any kind of German I'm familiar with... perhaps in some regional dialects such as Pennsylvania Dutch?
In my experience, Vieh is not used much and when it is, it's for large farm animals, especially cows -- but not e.g. for dogs or lions. (Unless you're being informal, in which case it may turn into Viech and can refer to pretty much any animal.)
The regular word for animal is Tier, that for a pet is Haustier (literally, "house-animal").
How can we differentiate the Das=the from Das=that?!
In writing? Only context. (In speech, the pronunciation will usually be different, as das "the" is usually unstressed but das "that" is usually stressed.)
Without context, as in a Duolingo translation exercise, both translations should generally be accepted.
How do you do tell the difference, when speaking, between -he is fast - "er ist schnell" and he eats fast- "er isst schnell"?
Only through context.
(At least in slightly careful pronunciation. At natural speeds, ist often gets worn down to iss while isst is less likely to lose its "t" in pronunciation.)