There are two main ways how you can import a new language into your new theme.
The first option - Add/Update a language
Go to your back office > Localization > Translations and find a tab named Add/Update a language. Here you can select the language you need to import and also the theme where the language should be imported. By this way you can also update translations.
All these translations come from the official Prestashop database. Some languages are translated 100%, some of them are still in progress. You can check the current state of translations of your language here: https://crowdin.com/project/prestashop-official.
If the quality of the translation of your language is too bad, you can try to search for it on google, there are many websites which offer these language packs, but usually they are not for free. The other option would be translating everything manually.
The second option - copy a language
In case you already use a theme with a language translation that meets your requirements, you can simply "copy" this translation into the new installed theme.
Go to back office > Localization > Translation and find a tab named "Copy".
Other option - import a language pack manually
Obviously, you can do this only if you have a language pack, most commonly when you purchase (or download for free) this pack from some website.
Let me know what was your experience with the installation of your language!