Hi, you can follow this steps: Installation instruction:

Open language archive and navigate to code >> extensions directory.
Now copy via (FTP) your_language (Slovak, Portuguese...etc..) the directory completely with content (folder) to >> EXTENSIONS folder on your AbanteCart.
Last step you can activate/use extension in admin. You will locate it in EXTENSIONS >> ALL EXTENSIONS section in Abantecart. There has all extensions, include the languages. Just click in "install" (green arrow) in your language (that should already appear there after you sent the folder via FTP)  then put the status "ON". You can see it in Abantecart website in "Extensions" >> "Languages".