When you launch Performance Dashboard you can specify the interface language by using the URL parameter language. If the argument is not specified at the URL, the language of the http request is used that results from the preferred language settings of the browser or of the operating system running the browser.
If the language cannot be identified either through URL parameters or via the http request, login dialog and dashboard list are displayed in English. In this case, the dashboard is displayed in the language of the first description XML element of the dashboard-specific definition file.
You need to ensure the the file webapp\assets\loginAppResources.xml contains the English resources because this language is used for displaying the login dialog if the language cannot be retrieved.