If you had more domains and languages, you would just create more config files. The standard config.toml is what Hugo will run as a default, but since we’re creating language-specific ones, you’ll need to specify each config file when running hugo server or just hugo before deploying.
So, for english and japanese versions I would have config_en.toml and config_ja.toml.
@askar You can rename config.toml to one of those versions that has the locale as prefix. You don’t need to have a file called config.toml, because the command --config="" allows you to specify a custom name for the config file.