[openstreetmap/openstreetmap-website] Fix double load of the language selector (PR #7371)

[Kai Michael Poppe]

### Description
Resolves intermittent SelectLanguageTest errors.

#### Problem
Opening the language selector loaded its Turbo Frame (`#select_language_list`) twice: once when the frame became visible, and again when `shown.bs.modal` rewrote `src` to add the current `source`. Every `turbo:frame-load` resets the search field, so the second (late) load could clear a query the user had already typed and the list snapped back to all languages. 
Besides the redundant request, this made `test/system/select_language_test.rb` flaky (the `"fra"` filter was wiped before the assertion).

#### Cause
- The frame auto-loaded via `src` (its `source` param baked in server-side, and stale after Turbo navigations).
- `shown.bs.modal` then set `src` again with the current `source` -> a second load, whose `turbo:frame-load` reset the search after the user could already interact.

#### Fix
Load the frame once, on `show.bs.modal` (before the dialog is interactive), with the current `source`, and only when it actually changes.

### How has this been tested?

Running the failing test in a local DEVCONTAINER a max of 30 times
```
for i in $(seq 1 30); do echo "run $i"; bin/rails test test/system/select_language_test.rb:6 || break; done
```
failed at the first the earliest or seventh the latest time. After adding to code of this PR this test ran 30 times without a problem.
You can view, comment on, or merge this pull request online at:

  https://github.com/openstreetmap/openstreetmap-website/pull/7371

-- Commit Summary --

  * Fix double load of the language selector

-- File Changes --

    M app/assets/javascripts/application.js (18)
    M app/views/layouts/_header.html.erb (2)

-- Patch Links --

https://github.com/openstreetmap/openstreetmap-website/pull/7371.patch
https://github.com/openstreetmap/openstreetmap-website/pull/7371.diff

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