Learn English – How to write a question with a suggested answer at the end

"Done." is a completely valid and grammatically correct "sentence." Technically a sentence must contain a subject and a verb, but the subject is implied and understood from the preceding discussion.

"Translation done." is not grammatically correct. At this point, I think it would be expected that you have a verb to complete the sentence. You sort of have to commit to one or the other.

After the comments, I'm updating this to explain why I would not say "Translation done." I've always been taught that "done," as a past participle, should not occur without an auxiliary verb. The exception provided for one-word sentences is accepted by most. But by adding another word without a verb it becomes a fragment, not a special type of sentence. If you find that acceptable, use it!

To me, "Translation done" is no more correct than the following:

I done.

It done.

We done.

"Mission accomplished" is a complete sentence, not a fragment.

None of this is to say that the recipient of your email wouldn't understand what you meant. In an office setting, I wouldn't hesitate using "Translation done." But I would do so knowing it was not grammatically correct.

But to your question, you can use any punctuation available to you in normal sentences.

1. Period

   Done.

2. Exclamation point

   Mary!

3. Question mark

   Where?

Because you have been taught an oversimplification.

Most English speakers would have no idea what you were talking about if you mentioned the "interrogative mood". People put a question mark on the end if it feels like a question.

Conversely, a polite order like

Would you sit down.

or a less polite one like

Will you sit down!

are often written without question marks, because although syntactically they have the form of questions, they are not in fact questions at all.