Can I translate scientific papers ?
Ethically, I do not see a problem with this. If you reference the original work and make it clear that it is translated then I think it should be fine.

The problem here is from a copyright point of view. Many journals are subject to copyright and if you post the findings online then someone could not pay to read the documentation and just go to your website and then translate it into whatever language they wish. 

I must mention that even though you think you're doing something that benefits others, without even making profits, you can still be in trouble. 

If you're really interested in translating a particular piece of work you would need permission from both the author and the publisher (if you're going to publish it online). 

My advice would be to ask permission from authors who have unpublished work. An example in finance is John Cochranes guide to PhD writing, this was translated from English to Chinese and placed on his personal website with credit given to the translator. 

This way, you don't get in any trouble and you are still gaining the benefits of translating that you mentioned in the question.