Site translate or multilingual

1. If you want to translate your website to another language:

- The fist step to change the default language of your site is go to Admin > Settings > General > Site Language: choose your language in the drop down list. However, this action only translate the core of WorldPress, you still need to install another plugin to help you translate the external themes and plugins you have installed.

- Bellow is a popular plugin will help you translate your site easily:

Loco Translate https://wordpress.org/plugins/loco-translate/ Free

Check its detailed documentation here to know how it works https://localise.biz/help/wordpress/translate-plugin/beginners

2. If you want to build a multilingual WordPress site:

You can install one of two following plugin, they all allow you to create a bilingual or multilingual WordPress site. Check their documentations to know more about usages.

- Polylang https://wordpress.org/plugins/polylang/ Free

Documentation: https://polylang.wordpress.com/documentation/

- WPML https://wpml.org/ Paid

Documentation: https://wpml.org/documentation/getting-started-guide/

Last but not least, this is some instructions which we think you could find it useful.

https://codex.wordpress.org/Multilingual_WordPress

http://www.wpbeginner.com/beginners-guide/how-to-easily-create-a-multilingual-wordpress-site/