Yes, you can. However, if you have already setup the store, we do not recommend to change default language.
The original text and translation text are two separated entities, they are linked to each other, but they can not 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 is that your website visitors would not know which language is the default. They only know your store support 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 settings, 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 you website in French by default.