Is it possible to translate the Input control validation message and report empty message to Swedish or any other language that is not present by default in JasperServer?

Requirement: 

Is it possible to translate the input control validation message and report empty message to Swedish, Please see the attached screenshot.

Recommendation: 

The two messages - 'This field is mandatory so you must enter data' and 'You must apply input values before the report can be displayed' are defined in the  jasperserver_messages.properties file - so it should be modifiable easily by language with locales as mentioned above or by just changing that particular file.

If after this, you see accented letters: 

The accented characters should be unicode escaped in the properties file. Otherwise,
ä =  ä
and
å = Ã¥

So, I believe that is what is happening. If you refer to the guide:
https://community.jaspersoft.com/documentation/jasperreports-server/tibco-jasperreports-server-administration-guide/v8/creating-locale/It is stated as follows:
The resource bundles described in this document consist of locale-specific Java properties files. Java properties files use the ISO-8859-1 (Latin-1) encoding that is the same as ASCII for all English non-accented characters. For international characters that are not in ISO-8859-1, use Unicode escape sequences (for example u00e9 is é).

Therefore in all the translation properties files, you can look up the unicode escape equivalent and use that instead of the accented characters. You can easily look at other property files such as es, and fr to see how they are done. You can google to find the Unicode sequence for the accented characters and use that instead.