Multilanguage: UI broken after saving page + file extension broken when saving page in second language: default.md.fr.md

When saving a new page in a multi-lingual site, the Admin UI is broken.

Steps to reproduce:

- Create a fresh install from zip with Grav 1.7.33. + Admin
- Open Admin
- Create new page and save: **All is well**

- Add languages `en` and `fr` in Configuration panel
- Create new page and save: **Faulty page layout**

- Save current page as second language: **Page throws error**

- Inspect the filesystem: **Faulty file extension**
```
user/pages/
├── 01.home
│   └── default.md
├── 02.typography
│   └── default.md
├── 03.pageone
│   └── default.md
└── 04.pagetwo
├── default.en.md
└── default.md.fr.md <-- Note the file extension
```