Both the interface and content language can be changed to suit your needs.
Interface
The Interface Language is used to change the language throughout your site. The following interface languages are currently available: English, 中文 (简体), Español, Português, Deutsch, Euskara, Français, Italiano, 한국어, Nederlands, Polski, Русский, Türkçe, and Tiếng Việt.
The interface text is stored in language files which are collected in the themes themes/hugo-nsd/i18n/ folder, as well as an i18n/ folder at the root of your project.
To edit the interface text, copy themes/hugo-nsd/i18n/en.yaml to i18n/en.yaml (relative to the root of your website). Open the new file and make any desired changes to the text appearing after translation:. Note that the language files are formatted in YAML syntax.
To translate the interface text to another language, follow the above instructions, but name the new file in the form i18n/X.yaml where X is the appropriate ISO/RFC5646 language identifier for the translation. Then follow the brief instructions in the Language section at the bottom of your config.toml. To change the default language used by the theme, set defaultContentLanguage to the desired language identifier in your configuration file.
To translate the navigation bar, you can edit the default [[menu.main]] instances in config.toml. However, for a multilingual site, you will need to duplicate all of the [[menu.main]] instances and rename the new instances from [[menu.main]] to [[Languages.X.menu.main]], where X is the language identifier (e.g. [[Languages.zh.menu.main]] for Simplified Chinese). Thus, the navigation bar can be displayed in multiple languages.
Content
To translate a content file in your content/ folder into another language, copy the file to filename.X.md where filename is your existing filename and X is the appropriate ISO/RFC5646 language identifier for the translation. Then translate the content in the new file to the specified language.
If a page has been translated into multiple languages, a language chooser will appear in the navigation bar to allow the user to select which language they would like to view the page in.
For further details on Hugo’s internationalization and multilingual features, refer to the associated Hugo documentation.