When asking for a translation of a sentence, what is the proper way to title the question? Should the sentence be included in the title? Or is a simple "please help me understand this sentence" or something of the like acceptable?
I recommend putting as much information as possible in the title. This makes the question stand out as distinct from other translation requests.
If everyone posted "Please help me understand this", then we'd have dozens of questions with the same title. This is bad for a couple of reasons:
Potential answerers won't see the actual question until they open the topic. Putting key words into your title helps draw attention so that someone who knows the answer can glance at the question list and go "oh yeah, I know that word! Let me help!"
Searching isn't as effective. Perhaps someone has the same, or a similar question and they try to search the web or JLU to see if someone else has asked their question. Well, the more obfuscated the details are, the harder it is to find the question through search! Especially through Google, where the question's title is used as the title of the result. Results titled "Please help me understand this" would probably not only rank lower but also look less likely to provide an answer than a page called "What does 日本語 mean in English?"
The related questions list becomes less useful. If you've ever taken a look at some of the related questions listed for a generic question title for a popular topic on StackOverflow, you can see this in effect. Someone could look for an answer, stumble across your question, find out it wasn't quite what they were looking for, so they would try to look at the related questions list (well, in an ideal situation anyway). But, if the related questions all have the same name, it requires manually inspecting every question in hopes of finding the desired answer, which even our ideal visitor will probably quickly give up on.
This doesn't apply just to translations either. In general, the more information supplied in a title, the more useful the title is to other people, including future visitors.
The actual sentence, if it is short. As here.
A snippet of the sentence indicating the part which is causing difficulty, as here.
If either of those are not practical for some reason, some indication of what the sentence is about would help - something about the source of the sentence, the dialect being used, the topic, etc. "Please help me with this sentence from Gintama." and "Please help me with this sentence about Ikebana."