The viewer interface can be localized by pointing its Localization parameter at a Stimulsoft language XML file.

Set the localization file.
<StiBlazorViewer Report="@Report" Localization="Localization/pl.xml" />

How it works. The viewer loads its captions from the specified file, so switching language is a one-attribute change.
На скриншоте ниже Вы можете увидеть результат выполнения данного кода:

Localizing the Viewer