Log in to Chromebook as the student.
Open Chrome.
In the URL bar, type chrome://settings/languages, hit enter.
Click top pane that says Language English (United States).
By default, you should see English (United States) and English.
Remove English (and any other extra languages here)so you just have English (United States) left.
You can remove non default languages by clicking the 3 dot menu button on the right.
Then click Add languages button.
Search for the language you want to add and check the box, and click Add.
Once you have added the preferred language, click the 3 dot menu beside it and check Show system text in this language.
Once you set the new language, a Restart button will appear. Click that and the Chromebook will restart in the preferred language.