Add translation for Japanese

[chasyumen]

No description provided.

[rhld16]

Thanks for submitting this PR! Just had a quick scan through and I'd love if you could take a second look over a couple things. Bare in mind this review was done using a mix of online translators so apologies if there are obvious linguistic oversights.

backup-file-upload-start - no mention of upload in any translations
backup-local-file-complete & backup-local-file-start - make sure to specify they're local backups
backup-method-not-auth - gold tag is not escaped
backup-method-prune-failed - specify the files selected to be deleted were due to exceeding the limit
backup-method-uploading - is file plural?
date-format-invalid - please specify UTC
external-ftp-backup-blacklisted - mention backup is from (S)FTP server
list-last-delimiter - are these no phrases used to end a list?
local-backup-limit-not-reached - does this phrase have the correct negation?
local-backup-no-limit - this may be set by choice, not necessarily only if the config is not set