You will help me with extending the localization support in the project.
The new locale is jp-JP, which I will further refer as $LOCALE.

Directory crates/i18n/assets contains the localization files in the Fluent format, each subdirectory representing one locale.
Add a localization support for $LOCALE by doing the following:
* create crates/i18n/assets/$LOCALE directory.
* copy the crates/i18n/assets/en-US/main.ftl into this directory.
* translate ALL entries in the copied file to the target locale $LOCALE.
* add a language-$LOCALE entry in there.
* modify all existing localization files and add a correctly translated language-$LOCALE entry in each of them.

Keep the original file structure intact, grouped by the key type. Do not modify the existing translation strings.
