Admin UI localization not working correctly for locales with country suffix

## To Reproduce

1a. **Without Assembly** create a localization with
```js
// sites/modules/@apostrophecms/i18n/index.js
module.exports = {
options: {
locales: {
en: {
label: "English",
// The default locale since there is no prefix requirement.
},
"de-de": {
label: "German",
prefix: "/de-de",
},
de: {
label: "German",
prefix: "/de",
},
},
},
};
```

1b. **With Assembly** simply create the two "German" locales via the UI.

2. Now simply log into your apostrophe site to see the admin UI. You should see the german version for "de" but for "de-de" it seems to be English, even though the localization for the template content is working fine.

## Expected behavior
German version of the admin UI for "de" and for "de-de"

## Describe the bug
It seems that the admin UI is translated differently than the template content. That leads to the bug, that only "de" has a german admin UI and "de-de" is English.

## Details

**Version of Node.js:**
14.18.0

**Server Operating System:**
MacOS X

**Screenshots**