Fanfiction is property of the author and any reposting or translations posted under a different name is classified as plagiarism.  Simply listing the genuine author's name in a disclaimer or footnote is adding insult to injury.  If you wish to see a story posted on a different site or a translation available, then contact the author and ask them to post it.  If you are volunteering to translate, then YOUR name should be the footnote since you were providing a service and not the creativity and intellect behind the creation of the piece.  

Fanart posted in any altered or unaltered form without permission is unacceptable.  If the artists has granted permission for you to manipulate their work, then you MUST credit that author.  

Just because the author/artist does not speak or read your language does not mean it is acceptable for you to take advantage of them.  If you wish to share their work with your peers, link to it.  If there is any way it can be misconstrued that the posted piece is yours, then you will be classified as a thief.  

 More authors/artists have stopped writing or drawing because of theft than you  realize.    Having to spend your time and energy fighting thieves instead of creating new works is enough to make this hobby unappealing.  It is a hobby, this is free entertainment provided to you by other fans and if you have any respect for their work then you will not distribute it as your own.  

Thankfully this only applies to a slim portion of our fandoms, however it is important for everyone to consider.  If you write/draw or simply appreciate the fanworks of your community than you will remain aware of this growing threat.  If you see stolen fanworks, please notify the author immediately so action can be taken.  If your work has been stolen, don't hesitate to ask for help.