[i18n] Overhaul the Japanese locale

[shgumo]

Problem

• The Japanese locale still carried POEditor-era artifacts: mistranslations (e.g. production settings rendered as "deployment settings"), literal translations that hid what features do, and 16 untranslated keys
• Two plural strings used a full-width pipe (｜) and rendered literally in the UI
• Terminology and notation were inconsistent across screens (production alone had five different translations)

Solution

• Fixed the broken plurals and mistranslations, translated all remaining keys
• Rephrased literal strings after verifying each feature's behaviour, and normalized notation throughout