Language Install: Check if WordPress has access to the filesystem without asking for credentials

[ocean90]

Currently the language chooser is displayed even when WordPress has no access to the filesystem. We should skip it in this case.

Related: #29395

[johnbillion]

It'd be nice to display some sort of error message in this case, but it may be unnecessarily confusing for a new install. On the other hand, it would be confusing if a user was expecting to be able to choose their language here and saw no means to do so, yet saw no error message either.

[nacin]

The UX in this patch is what I intended — it would skip the process if the FS or API wasn't available. So, this is good. I might want to study the function name but it's good otherwise.

[nacin]

Can we also resolve the @todo in wp_download_language_pack()? Should be just simple enough to check the return value, I think.

[ocean90]

In 29673:

Install: Only show the language chooser when we have access to the filesystem without asking for credentials.

fixes #29397.