Add Japanese localization (ja)

[era760]

I've added the Japanese translation (messages_ja.properties) and updated Labels.java.

• Followed professional networking terminology (e.g., using "Netmask" and "IP Address" in English for clarity).
• Applied standard Japanese UI conventions for access keys like (&A).
• Verified the build and passed all tests locally.
• I’ve only localized the most basic UI elements into Japanese.

Hope this helps Japanese users! Thanks for the great tool.