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If you're wanting to change the language that your store is in, you can easily make this change at any time by going to Online Store > Themes > Actions > Edit languages. In here, you'll see an option right at the top of the page to "Change theme language." Here's a screenshot example of this:
When you click on that, you can choose the main language that you want to display your store in. If the majority of your client base will be in a specific country in a particular language, then having it show in that language can really be a benefit for your business!
You can also install apps or widgets to your theme so your customers can select which language they'd like to view your store in, and here are a few options for that:
Hopefully that helps! Let me know if you have any further questions that I can help you out with.