Implement page translation fallback

[nlubisch]

1. Why is this change necessary?

2. What does this change do, exactly?

3. Describe each step to reproduce the issue or behaviour.

4. Please link to the relevant issues (if any).

5. Checklist

• I have written tests and verified that they fail without my change
• I have squashed any insignificant commits
• This change has comments for package types, values, functions, and non-obvious lines of code

[codecov]

Codecov Report

Merging #718 into development will increase coverage by <.01%.
The diff coverage is 7.59%.

@@              Coverage Diff               @@
##           development    #718      +/-   ##
==============================================
+ Coverage         5.43%   5.44%   +<.01%     
==============================================
  Files              257     259       +2     
  Lines            15997   16056      +59     
==============================================
+ Hits               870     874       +4     
- Misses           15127   15182      +55

Impacted Files	Coverage Δ
htdocs/src/Oc/Page/Persistence/BlockEntity.php	100% <ø> (ø)
htdocs/src/Oc/Page/Persistence/BlockService.php	100% <ø> (ø)
htdocs/src/Oc/Page/Persistence/BlockRepository.php	0% <ø> (ø)
htdocs/src/Oc/Page/Persistence/PageEntity.php	100% <ø> (ø)
htdocs/src/Oc/Page/Persistence/PageRepository.php	0% <ø> (ø)
htdocs/src/Oc/Page/Persistence/PageService.php	100% <ø> (ø)
htdocs/src/Oc/Page/PageProvider.php	0% <0%> (ø)
htdocs/src/Oc/Page/Controller/PageController.php	0% <0%> (ø)	⬆️
...src/Oc/GlobalContext/Provider/LanguageProvider.php	90.9% <0%> (-9.1%)	⬇️
htdocs/src/Oc/Page/PageStruct.php	0% <0%> (ø)
... and 5 more

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