Yes, you can. However, if you have already set up the store, we do not recommend changing the default language.
The original text and translation text are two separated entities, they are linked to each other, but they cannot be interchanged. For example, if you change the default language from English to French, the French translation text will not automatically replace the original text which is in English. You would need to manually input the French text into the Shopify admin.
If you want to change the default language because of website visitors, we would like to point out that your website visitors would not know which language is the default. They only know your store supports multiple languages such as English and French.
To offer a better user experience, you can use the language selector offered by our App and set it to use "Customer's browser language" as default display language, as shown in the screenshot below.
With this setting, if the website visitors' browser language is English, we will display your website in English by default to such visitors. If the website visitors' browser language is French, we will display your website in French by default.