According to Admin Guide for SAP BW,
a) The texts in the design tool user interface are available in English only.
b) The texts of the data from the connected SAP NetWeaver BW system (such as master data and metadata of the selected data source) are language-dependent and are displayed in the default language configured in the user settings.
c) Message texts and tooltips in the executed analysis applications are available in the following languages: DE, US, ES. FR, JP, BR, RU and CN based on the rule outlined in the guide.
So, if your datasource is SAP BW cube/query, you can leverage the language dependent master data and metadata.
If your components are bound to BW data (like listvalues of dropdown, radiobutton etc), they probably will be displayed according to the user language.
As far as setting the textboxes to user language, I don't think it is doable with CSS alone.