TranslatePress

1. Set up your languages in Settings → TranslatePress → General.
2. Tick the same languages on the Languages tab and choose the same default language.
3. Add the switcher and turn off TranslatePress’s floating language switcher if you used it.

The plugin builds each link with TranslatePress’s URL converter, so translated slugs and your language URL slugs (for example /fr/ or /francais/) are used. If you turned on Use a subdirectory for the default language, that is followed too.