If you have the Autodesk Desktop App installed (it usually runs in your system tray), it often detects language updates automatically.
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If you have a customized acad.cuix created in English, loading it while running a Japanese UI will result in mixed languages. The core UI switches to Japanese, but your custom toolbar remains English. You must migrate CUIx files using the tab in the CUI editor. If you have the Autodesk Desktop App installed