[Enhancement] Cleanup of code and text

[rudosch]

I took some time to look at the code and text and so far can offer the following corrections:

• fix minor spelling and grammar errors
• fix French translation file /main/resources/i18n/text_fr.properties
• add missing response.setContentType for some servlets
• fix some deprecation warnings, e.g. new Integer() -> Integer.valueOf()

Some other deprecation warnings are directly related to the use of library json-simple-1.1.1.jar (compiled for Java 1.2!),
I could try to replace related code to use library json-20220320.jar already in use at other places.