While you might not be able to change the menu text, you can often change the for DVDs:

While a disc is playing, touch the screen to bring up the playback menu.

Look for the Audio Language icon (often a speech bubble or "Audio") to cycle through available tracks.

Loading unverified firmware can "brick" your device (make it permanently unusable).

For units that are strictly Japanese, some enthusiasts use third-party "English conversion" SD cards or firmware updates.

On models that do support English, the process usually follows this path: Press the or Info button on the physical unit. Look for the icon resembling a gear (Settings/Setup). Navigate to the System (システム) or General menu. Find the Language (言語) option.

Select "Translate" to see the Japanese text converted to English.

If "English" appears, select it and confirm. You may need to restart the car for changes to take effect.