We wrote a few guides to help you.

We recommend:

• WPML (paid) - For creating translations of your content and theme strings.
  • WPML Flags in Top Menu
  • Installing & Setting up WPML
• Polylang (Free) - For creating translations of your content.
  • https://wordpress.org/plugins/polylang/
  • Polylang Error | Warning: Illegal offset type in isset or empty
  • Using Polylang
• Poedit (Free) - For creating translations of your theme strings manually. 'Read more', 'Search' buttons etc.
  • Translate Strings using Poedit
• Loco Translate (Free) - For creating translations of your theme strings via dashboard. 'Read more', 'Search' buttons etc.
  • https://wordpress.org/plugins/loco-translate/

Cheers!