Language code in resource filename kept in RelPermalink

As in the example below:

```bash
index.md
style.en.css
style.fr.css
```

The language indicators tells Hugo what language/site they belong to, but currently these would be published to `https://example.com/style.en.css` and `https://example.com/style.fr.css`.

The above was never intended, and probably the reason why we did not discover #7436 