Add translation for the frontend in Lab

[fcollonval]

Plug in the translation infrastructure of JupyterLab for the frontends.

[krassowski]

Some untranslated strings I noticed from using the labextension:

• 
  

  nbdime/packages/nbdime/src/diff/model/notebook.ts

  Line 41 in f044d23

  this.metadata.collapsibleHeader = 'Notebook metadata changed';

  
  (note: in the diff mode, it is fine in the merge mode)

• 
  

  nbdime/packages/nbdime/src/common/mergeview.ts

  Line 468 in f044d23

  outer.textContent = view.state.phrase('(…$ unchanged lines…)', this.lines);

• Lab commands:
  
  

  nbdime/packages/labextension/src/plugin.ts

  Lines 199 to 200 in f044d23

  	label: erroredGen('Notebook diff'),
  	caption: erroredGen('Display nbdiff between two notebooks'),

  
  

  nbdime/packages/labextension/src/plugin.ts

  Lines 224 to 227 in f044d23

  	label: erroredGen('Notebook checkpoint diff'),
  	caption: erroredGen(
  	'Display nbdiff from checkpoint to currently saved version',
  	),

• Server missing message
  

  nbdime/packages/labextension/src/plugin.ts

  Lines 38 to 39 in f044d23

  	const serverMissingMsg =
  	'Unable to query nbdime API. Is the server extension enabled?';

[fcollonval]

Thanks Mike for looking at those

As you mentioned, some will be hard to solve like the one using the CSS content for hidden cells. And I'm uncomfortable to translate the collapsibleHeader as it is defined in the model - I prefer the translation to occur only in views. But I could probably go ahead as changing the code logic will be too large.

[fcollonval]

Thanks @krassowski after 6ccc4cc the element remaining in Lab is the unchanged cells as it is set in the CSS. The web apps template are also not covered as anyway it will require quite some work to add server support for handling the mo/json files.

[krassowski]

Thanks! I moved the remaining bits to #724 (comment).

The snapshots will need updating after the italics were removed. I would recommend merging with main to pickup #714 that I just merged to facilitate.

[fcollonval]

bot please update playwright snapshots

[github-actions]

Playwright windows-latest snapshots updated.

[fcollonval]

Kicking the CI

[krassowski]

Seems only Windows snapshots were updated:
bot please update playwright snapshots

[github-actions]

Playwright ubuntu-22.04 snapshots updated.

[krassowski]

Case in point that the background changes are too subtle to be picked up @HaudinFlorence. The snapshot on the left was not updated in #714 and all tests were green because as highlighted in #714 (comment) the defaults are too lax to reject on subtle colour changes.

[krassowski]

Thanks @fcollonval!