If you can't wait for the next official JJazzLab release, you may want to try JJazzLab with the latest translation files, to see how it looks.
It's not that simple, but perfectly feasible if you're motivated enough. Below are the steps to follow.
If not, install it from adoptopenjdk.net.
This is the development environment and platform on which JJazzLab-X is built.
You need to patch Netbeans 12.0 to overcome an annoying limitation regarding the file encoding of language files.
Download the patched file org-openide-util.jar
Go to <Netbeans_install_dir>/platform/lib
Replace org-openide-util.jar with the patched version (for safety make a backup of the original file)
Enter repository address https://github.com/jjazzboss/JJazzLab-X.git
Leave user and password blank
Press Next and select only the branch l10n_master (l10n means "localization")
let Netbeans open the JJazzLab-X project from the cloned files
Select the JJazzLab-X project, right-click Build. When it's done you should see "BUILD SUCCESSFUL" in the Output window.
Select the JJazzLab-X project, right-click Run.
JJazzLab-X should start with the default language set up for your computer, e.g. if you use Windows in the German language, you should see JJazzLab in German. If a source phrase is not translated in the target language, the English one is used.
You can't switch the language from within the JJazzLab-X application when it is run from Netbeans IDE. But it's still possible to change language for test purposes:
In Netbeans, Projects tab, open the JJazzLab-X project, then the Important Files folder
Open Project Properties
Find "run.args.extra=..." and insert "--locale xx:XX " right after the "=" For example: "run.args.extra=--locale fr:FR \"
Use fr:FR for French, de:DE for German, en:US for English, it:IT for Italian, es:ES for Spanish, ja:JP for Japanese, zh:CN for Chinese (mandarin).