pdfViewer toolbar are always in english :
I assume the reason for this is that the localization file chosen have no messages for the PDFViewer. Note that the localization is a contribution-based resource - https://docs.telerik.com/kendo-ui/globalization/localization#contribution. You can check some examples for PDFViewer localization here: https://github.com/telerik/kendo-ui-core/blob/master/src/messages/kendo.messages.zh-CN.js#L1700-L1762.
The mechanism for the localization works and you can test it out with the messages you would like to add here: https://dojo.telerik.com/ozIbUliy.