Installation instructions in English:
Download the game from Steam or ichi.moe.
Download the translation:
https://github.com/proudust/ddlc-jp-patch/releases/download/190801/DDLC_JP_190801.zip
(also linked on the japanese Steam mod page)
Open the zip, go into the game folder, then copy both of the .rpa files into your DDLC/game folder.
To find your DDLC game folder on Steam:
Right click Doki Doki Literature Club → Manage → Browse local files
The game started in Japanese automatically for me, but if not, you may have to select the language in the settings.
Also, here’s a guide to set up very useful dictionary/translation aids for the game: