[UX] Installer: Allow using .po translation files from d.org without having to rename them.

[klonos]

@quicksketch in #2217 (comment):

...we don't have adequate documentation in the first place for this. Right now we link to http://drupal.org/localize for instructions when you click "Learn how to install Backdrop in other languages". But the instructions for Drupal aren't the same as they are for Backdrop.

As for this exact problem, it looks like the file needs to be renamed from drupal-7.50.de.po to install.de.po then Backdrop will be able to find it.

@olafgrabienski in #2329

When I try to install Backdrop in German following the installer instructions and @quicksketch's hint in #2217 (comment) about renaming the language file, ...
...

• Download the current Drupal 7 German language file (http://ftp.drupal.org/files/translations/7.x/drupal/drupal-7.51.de.po), rename it to install.de.po, and put it in files/translations.

The translation files available from https://localize.drupal.org/download are in the format drupal-%core_ver%.%langcode%.po, we should be accounting for these filenames when scanning for language files instead of making the users do additional work.

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PR by @klonos: backdrop/backdrop#1649

[klonos]

@jenlampton before you get upset at me for (ab)using the bug report tag, If this issue had a title of "Translation files downloaded from d.org are not picked up by the installer", then it would be one 😜

[jenlampton]

:)

[quicksketch]

I feel like this is a good temporary measure, it'd certainly help with new installations and people coming from Drupal until we can provide our own localization server. I think adding a @todo: Remove "drupal" check in 2.x. would be appropriate here (assuming we have a localization server by then).

[klonos]

@todo added. Sorry for taking ages to respond to PR reviews.

[olafgrabienski]

I just tested the PR locally using current Backdrop dev and the patch by @klonos . I was able to install Backdrop in German without renaming the .po file from drupal.org. Great!

[quicksketch]

Thanks @olafgrabienski and @klonos. Merged backdrop/backdrop#1649 into 1.x and 1.5.x. Thanks!