You can convert Japanese language text into speech with the software called Balabolka [It’s a Russian made open-source software]
Step 1: Add JAPANESE language in your Windows language settings.
Step 2: Download the Balabolka software
Step 3: Open the software and go to the SAPI 5 tab and select HARUKA from the drop-down menu
Step 4: Write any Japanese content (no restrictions for word count) and preview the voice, then export it as .mp3 file.
BALABOLKA mean Chatterer in Russian