You can change your site main language via Settings -> General -> Site Language
It’s easy to localize your site using Loco Translate plugin. Here you can see related tutorial - https://localise.biz/help/wordpress/translate-plugin/beginners.
Another way is to add your translations via creating your own .po files. Before adding translations for static strings it needs to rename language files correctly according your site languages. Theme language files are located in a ../wp-content/themes/theme-folder/languages/ folder. You can find a list of language codes at WordPress in your Language. As an example, the language code for English is en_EN, so you would save the translated files as en_EN.po first, then en_EN.mo.
There are several ways to fill up translations. One of them if using PoEdit platform. You can add translation via editing your .po file and at the end PoEdit generates .mo file for you. Then you needs to upload ready translation files to primordial folder (../wp-content/themes/theme-folder/languages/).