Custom reports can be localized to support the different language and data format requirements.
Copy your custom report template and name the copy according to the following standard:
[TemplateName]_[LanguageCode].rpt or [TemplateName]_[Language-CultureCode].rpt
For example, for English, the custom report template would be named [TemplateName]_en.rpt. For American English, the report template would be named [TemplateName]_en-us.rpt.
Perform this step for each localized version of your report template that you need.
In this report template, make any changes you need to support the proper localization of data for the targeted locale.
Rebuild your custom report assembly and copy it to the SPFServer\bin\ReportLibraries folder.