How to translate a theme after adding a new default language?

What you are seeing

You added a new language in your Shopify admin and set it as the default one, but your storefront is still in the old language. Buttons, labels and menus have not changed, and running an auto translation does nothing.

Why it happens

The language you want translated is the default one. The Translate & Adapt app by Shopify does not translate the language that is currently set as default, because it treats it as the source, not the target. This is the usual cause.
The translation app is not installed yet. Adding a language in Settings > Languages publishes it, but nothing translates the theme text automatically.
The theme text was never translated, only the products were. Product titles and descriptions are separate content from theme text such as Add to cart or Search.

The fix is to move the default temporarily to another language, translate the one you want, then set it back as default.

Step 1: Set a different language as default

Steps:

Install the Translate & Adapt app by Shopify.
Go to Settings > Languages.
Click Change default and pick any other language as the default.

Result: the language you want to translate is no longer the default, so the app can now treat it as a target.

Changing the default removes your previous language entry from the list. Add the language you want to translate back in Settings > Languages and publish it before you continue.

Step 2: Auto translate with Translate & Adapt

Steps:

Go to Apps in your Shopify admin and open Translate & Adapt.
Select the language you want to translate.
Click Auto-translate.

Result: the app fills in the theme text for that language. Larger stores can take several minutes.

Step 3: Set the translated language back as default

Steps:

Go to Settings > Languages.
Click Change default.
Select the language you have just translated and save.

Result: your storefront now opens in the translated language for every visitor who is not routed to another market.

Shopify shows a warning at this point about changing the default language. It is expected here, because you are switching back on purpose. Save your change.

When to contact support

Contact support if some theme text stays in the old language after a successful auto translation, or if a translated string breaks the layout (text overflowing a button, for example). Translation quality itself is handled by Shopify, not by the theme.

Send this with your first message:

Your store URL, and the URL of a page where you can see the untranslated text.
Which language pair is involved, and which language is currently the default.
A screenshot with the untranslated text circled.
Theme name and version: Laurel 1.0.0, shown under the theme name in Online Store > Themes.
A staff account for the support team if they need to look at the theme content themselves.

Theme translation : translating into one language, or selling in several at once
Change currency : the currency side of selling to more than one country
Staff account: how to give the support team access to your store