To configure the language of OBS itself, configure the language in OBS.
To configure the language of the browser docks pulled from Twitch, configure the language in Twitch.
There is a 3rd language involved in web browser sessions, this is the language of your browser. As default, the OS system language is used for this. If your Windows primary language is Japanese, and the website is language-aware, it might read the browser setting and appears in Japanese.
In Chrome, you can configure the language in Settings->Advanced. However, I don't know what language the OBS-internal browser will use that is used for the docks. May be the OBS language, but may also be the OS language.