This might be off topic, but how well should one know a language or technology to put it on a resume.
Depends on your conscience
Seriously, feel free to put anything on your resume, as long as you are easily able to quantify it when probed. For example, if a short and curt resume, if you only put programming languages as bullet points like so:
* Java
* C++
* C#
* Scheme
* Ruby
... then be ready to quantify it when asked, such as "oh, Ruby, I had several months experience working on it when I was part of a team that used it to make an enterprise-wide logistics system - about 300 concurrent users or so. I was in charge of developing the framework and the API so that the grunt developers can churn out and maintain code faster."
Don't make things up, be honest, even if it means just saying
"Oh, about Perl, I've been using it for about 6 months for small personal projects like cash flow tracking and report generation based on expense type, etc, nothing really big or enterprise-scale so far". Of course, if you had space in your resume, you could save them the trouble and put a note alongside it, like so:
Programming languages used in my career:
*Qbasic - 2 years, mostly grunt dev and maintenance work for small payroll systems
*Turbo Pascal - 1 year, just small personal projects including DOS-based calendar app.
*C - 10 years, one of the key developers of the Linux kernel; yeah, that's right, the goddam Linux kernel
Definitely, the languages you are best at should be the one you highlight. It does no harm to put all other languages or technologies you know, even in passing, as long as you don't make it look like you've mastered all of them as much as the languages you are actually very skilled at.
It becomes a very awkward scenario when someone claims guru-like expertise on his resume for something like, say, PHP, and then does not even have a clue that
$$variable is legit but doesn't work like a normal variable (true story). Better yet, someone listed Python in his expertise (just one among many, about 10 or so), and then somehow couldn't figure out why a sample code got borked when I messed with the spacing. It was a Twilight Zone moment (for him) and eventually commented that maybe the compiler is bugged and I should upgrade my Python to the latest version, and (because the test machine was my laptop, which back then was running Fedora or Ubuntu) further commented that, as far as he could remember, Python was much more stable in Windows than in Linux. Yeah, right. In the end, turns out he is not skilled at Python at all (no surprise there), but just had one college course about it, the promptly never used it again in his life.
Anyway, you get the message. Put anything you want there, but be honest with the assessment because getting caught is embarrassing and career/rep-damaging.