Storyline 360 - Report.html

Our team was just doing similar research recently. There are some limitations there.

1. You can only have one single Report.html under your program folder. So, assuming you have customized it, but when you need the report to be in target language(s), the publishing process will always read the "report.html". 
   So some workaround there, you can create multiple report.html, one for each target language. And then when you are to publish German course, rename report_de.html to report.html; when you are to publish Italian course, rename report_it.html to report.html....
2. Or, simply publish course and replace the file from the output folder.

If you are using the 1st approach, be aware that it looks to be only support ANSI or Windows-1250 encoding. I tried localizing the strings into German so there were some extended characters, or try Japanese  Chinese, once the publishing process is completed, the report.html will have garbled text.