Use Relative Permalink in Language Selector (#764)

### Prerequisites

Put an `x` into the box(es) that apply:

- [x] This pull request fixes a bug.
- [ ] This pull request adds a feature.
- [ ] This pull request introduces breaking change.

### Description

Makes the language selector independent of the baseUrl config. This is
helpful if e.g. multiple domains alias to the page.

### Issues Resolved

None, afaik.

### Checklist

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#### General

- [x] Describe what changes are being made
- [x] Explain why and how the changes were necessary and implemented
respectively
- [ ] Reference issue with `#<ISSUE_NO>` if applicable

#### Resources

- [ ] If you have changed any SCSS code, run `make release` to
regenerate all CSS files

#### Contributors

- [x] Add yourself to `CONTRIBUTORS.md` if you aren't on it already

layouts/partials/header.html

@@ -28,7 +28,7 @@
                 {{ $node.Scratch.Set "separator" false }}
               {{ end }}
               <li class="navigation-item">
-                <a href="{{ .Permalink }}">{{ .Language.LanguageName | emojify }}</a>
+                <a href="{{ .RelPermalink }}">{{ .Language.LanguageName | emojify }}</a>
               </li>
             {{ end }}
           {{ end }}