Is the JLPT heavily weighed when companies are considering their candidates? I know it helps to show your investment in Japan but I wonder to what extent do companies consider this.
Also what test level (N1 - N5) do Japanese companies consider fluent enough to work with them?
As described in getting your first programming job in Japan, the more Japanese you speak, the better your odds of landing a job.
Companies don’t care about whether or not you’ve passed the JLPT. They do care about how much Japanese you speak. While the JLPT can be a way to demonstrate you understand Japanese at a certain level, even if you haven’t taken the test, as long as you can demonstrate equivalent Japanese ability, it shouldn’t be an issue.
What level you should be aiming for depends on where you’re applying. For companies that don’t expect you to speak Japanese, but would like you to have an interest in learning, having any level is better than none. On the other hand, when companies are looking for someone who can already speak Japanese, they’re typically looking for a level roughly equivalent to N2.
I have never taken the JLPT. No Japanese person or company has asked me about the JLPT, either. I don’t think it has been a consideration in any of the interviews I’ve conducted, both my successful and failed ones.
As pwim notes above, they definitely care about how much Japanese you can communicate, both written and spoken.
If you want to take the JLPT, do it for you own reasons, but don’t do it to help you get a job. In my opinion, the best use of time would be to get help writing a really good 履歴書, practice 面接 with a teacher, and visit sites like qiita.com to really sink your teeth into communicating technical subjects in Japanese.
They’ll know in 5 minutes of conversation with you where you stand, a test (or there lack of) won’t help or hurt you compared to that.