Open Interface Preferences

Choose and select the Interface tab:

Select a language

Select the language you prefer:

Restart

Quit and restart to make the change in language take effect:

Use code captions

With code captions turned on, Wolfram Language function names are annotated with their translations. Hover over a truncated name to see the name in full:

To turn off code captions, uncheck Show code captions in the Interface Preferences:

Notes

The selected language is available to programmatic operations via $Language.