How can I put my store in maintenance mode?

You can put your store on the maintenance mode, in two steps with the ability to leave a message to your customers in each language activated on the store, to do so, please go to the General settings through the store settings:

Then, you will find the maintenance mode settings, once you enable its status, a field will appear for you to set a message in each language activated on the store to be displayed to your customers when they visit the store while it's in maintenance mode:

Finally, make sure to click on the "Save" button once you've done setting the maintenance mode settings.