To install a language pack for AutoCAD 2018, you generally need to download the specific pack from your Autodesk Account and run the installer. Note that AutoCAD must be installed first, as language packs are add-ons that allow you to switch the interface and documentation without reinstalling the entire core software. Steps to Install AutoCAD 2018 Language Packs
Since Autodesk removed direct links for 2018, your best options are: autocad 2018 language packs install
Navigate to All Products and Services and find the AutoCAD tile. Click View Details. Select the 2018 version and switch to the Languages tab. Select the desired language and click Download. In-Product Download: To install a language pack for AutoCAD 2018
acadui.dll for that language is valid.