UI text not translated when creating a multilingual site with only one locale

[sschultze]

What version of starlight are you using?

0.20.1

What version of astro are you using?

4.4.4

What package manager are you using?

npm

What operating system are you using?

Mac

What browser are you using?

Brave

Describe the Bug

If I create a multilingual site with only one non-English locale like this, the UI text (like "On this page") is not translated, but always English instead.

defaultLocale: 'de',
locales: {
    de: {
        lang: 'de',
        label: 'Deutsch'
    }
},

As soon as I add another locale, for example fr, the UI text of the de locale is correctly translated to German.

Why am I trying to create a multilingual site with only one locale in the first place? I want to prepare for translations in the future.

Link to Minimal Reproducible Example

https://stackblitz.com/edit/github-4oxm53?file=astro.config.mjs

You have to visit /de/ in Stackblitz manually. For some reason, the redirect from / to /de/ doesn't seem to work in Stackblitz - in my local installation, it does.

Participation

• I am willing to submit a pull request for this issue.

[delucis]

Thank you for the report and the clear reproduction @sschultze!

I can confirm I’m seeing the same thing in your StackBlitz link. I think we never accounted for this situation with the expectation that monolingual sites always used the root as the default locale (as shown in our i18n guide), but this should work.

[sschultze]

Thank you @delucis !

[rgilsimoes]

I did a little debugging and found out that the when returning the UserConfig it's expected to have "root" present but because of the Zod configuration it is not mandatory.

This would be fixed by changing this on user-config.ts:

	/** Configure locales for internationalization (i18n). */
	locales: z
		.object({
			/** Configure a “root” locale to serve a default language from `/`. */
			root: LocaleSchema.required({ lang: true }).required(),
		})

Either way I've also noticed that if we set "root" plus another locale setting the "defaultLocale" has no effect.