"There is almost no more milk in the glass."
In english you will never hear "almost no more", we would probably say "hardly any left"
Assuming this sounds more natural in French than in English, phrasing it this way in English at least guides us in the exercises where we have to translate from English to French.
Whilst that is true, it doesn't seem to hold for "Appeler un chat un chat" which duolingo is translating as "Call a spade a spade"
You're right, but I think the situation is a little different. The sentence here is something that I agree sounds mildly awkward or unusual in English, but if somebody said it, I'd know what they meant and I can even imagine it being said by a native English speaker. The way it's phrased here is closely related to the French phrasing, which certainly helped me to translate it correctly. Otherwise I'd have been inclined to translate into a "negative" French way of saying it, to refer to what Sitesurf has said.
The other example is an idiom, which is another beast entirely. In English we simply don't say "to call a cat a cat," but we have a completely analagous idiom which arbitrarily uses the noun "spade" instead of the noun "cat." I don't know why either of these objects gets used in either version of the idiom, but that's idioms for you.