Fix translator_setup: undefined default_translator_class for linkcheck builder

[lang-m]

The translator setup is called unconditionally to add the HootstrapHTML5TransatorMixin. If no translators have been set explicitly this falls back to app.builder.default_translator_class, which is however not defined for the linkcheck builder (maybe others), resulting in an attribute error:

$ sphinx-build -T -b linkcheck <src> <dest>
<...>
Traceback (most recent call last):
  File "/PATH/TO/venv/lib/python3.11/site-packages/sphinx/events.py", line 404, in emit
    results.append(listener.handler(self.app, *args))
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/PATH/TO/venv/lib/python3.11/site-packages/pydata_sphinx_theme/translator.py", line 88, in setup_translators
    app.builder.default_translator_class,
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
AttributeError: 'CheckExternalLinksBuilder' object has no attribute 'default_translator_class'

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/PATH/TO/venv/lib/python3.11/site-packages/sphinx/cmd/build.py", line 496, in build_main
    app = Sphinx(
          ^^^^^^^
  File "/PATH/TO/venv/lib/python3.11/site-packages/sphinx/application.py", line 295, in __init__
    self._init_builder()
  File "/PATH/TO/venv/lib/python3.11/site-packages/sphinx/application.py", line 369, in _init_builder
    self.events.emit('builder-inited')
  File "/PATH/TO/venv/lib/python3.11/site-packages/sphinx/events.py", line 415, in emit
    raise ExtensionError(
sphinx.errors.ExtensionError: Handler <function setup_translators at 0x7f73ec330400> for event 'builder-inited' threw an exception (exception: 'CheckExternalLinksBuilder' object has no attribute 'default_translator_class')

This PR adds a safe check for app.builder.default_translator_class and skips the translator setup if the attribute is not defined.

I have tested the modified version with linkcheck, but I am not sure where to add a unit test for these changes.

[github-actions]

Coverage report

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File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
src/pydata_sphinx_theme
translator.py					81-83
Project Total

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