Hey @TBS2023 !
You should be able to find your theme translation options by going to your Shopify admin > Online Store > Themes > click the … button, choose Edit default theme content and searching for the line you need to translate.
However, keep in mind that changing your Sold Out badge text to “Available for pre-order” won’t make the product purchasable as a pre-order item. You can tick the “Continue selling when out of stock” option on your sold-out products to continue overselling them, however this will remove the Sold Out badge, so it won’t be clear that this product is a pre-order.
I’d recommend using a pre-order app like Early Bird to manage expectations for your customers, as it can show them clearly that the product is a pre-order, shows a custom badge on products for you (so you don’t need to edit your Sold Out translation), and gives you a lot more options around partial payments, discounts and scheduling pre-order campaigns.
It also has a simpler back-order function which will show some basic info on the product page about the item being a back-order purchase, as well as swapping the Add to Cart button text and showing a badge to make this clear. This can be easily applied to your entire catalog of sold out items too.
Let me know if you had any other questions about pre-orders! Hope this helps 