In my opinion, the line between bulk translation requests (off-topic) and phrase requests (on-topic¹) is a rather clear one:
If there is a realistic chance (from an answer’s point of view) that the question can be answered by something that would make for a single item in a reasonable dictionary, it is on-topic¹. Or with other words: If it is conceivable that a language developed an idiom to express something, it is on-topic¹.
Thus, if there is a single idiom that answers the question, it is automatically on-topic¹. However, if there isn’t, this doesn’t make the question off-topic; the answer is just something along the lines of: “There isn’t any such phrase in the German language.” In my opinion, the question should just stay unanswered in this case. In particular, if there is an idiom in another language that expresses the desired meaning, it is conceivable that German has such an idiom as well and the question is on-topic.
How do I say: “There is a phrase for what I want to say, but I can’t quite remember it.”
This on-topic, because there is in fact an idiom, namely “es liegt mir auf der Zunge” that expresses the desired meaning. Even if the German idiom didn’t exist, it would still be reasonable for an asker to expect that there is such an idiom in German, because English has one, namely: “It’s on the tip of my tongue.”
How do I say: “Yesterday I asked a phrase request but the question was closed.”
This not on-topic, because there is no reason to expect that a single phrase expressing this exists.
¹ in terms of not being a bulk translation request. We may still be consider the question off-topic for other reasons, in particular the general-reference close reason, i.e., being easily answerable by a dictionary.