Using Java version 11, the display of the calendar week is bound to the country and not to the language. This leads to errors in the presentation of the calendar week, which did not occur with the previous Java V8.
Now, if you are using Java V11, JustRelate CRM provides a possibility to specify specific browser locales in the CRM web client. This is a combination of user settings and administrative defaults that gives good control over the locale to use for a particular CRM Web Client session.
The most important information about this can be found in the document How to specify particular locales in the Web Client (PDF).