Install a language selector app from the Shopify App Store. Some popular options include Langify, Weglot, and Translate My Store.
Once you've installed the app, go to your app settings and customize the language selector button to your desired design and position. Some language selector apps may also require you to add language translations for your website content.
After customizing the language selector, go to your Shopify theme editor and select the header section.
Insert the HTML code for the language selector button in the header section where you want it to appear. The HTML code for the language selector button will be provided by the app you installed in step 1.
Save the changes and preview your website to ensure that the language selector button is displaying correctly.