Plugin localization didn't fallback to language if locale is specified

[miketsoft]

Prerequisites

• I have searched for similar issues in both open and closed tickets and cannot find a duplicate.
• The issue still exists against the latest master branch of bootstrap-fileinput.
• This is not an usage question. I confirm having read the plugin documentation and demos.
• This is not a general programming / coding question. (Those should be directed to the webtips Q & A forum).
• I have attempted to find the simplest possible steps to reproduce the issue.
• I have included a failing test as a pull request (Optional).

Steps to reproduce the issue

We have a multi-language Yii2 application and use FileInput widget. But when the application language (locale to be more precise) Yii::$app->language is set, for example, to uk-UA, the plugin on the page is rendered with the default language (English).

While method \kartik\base\BaseAssetBundle::addLanguage attach to asset correct l10n file (uk.js), widget \kartik\file\FileInput register plugin with "language": "uk-UA". So, variable $.fn.fileinputLocales has key uk, but the plugin requests uk-UA and fallbacks to en.

The same problem is described in comment

Expected behavior and actual behavior

I was expecting, that when I set the locale for the plugin, it'll correctly fallback to the language part of the locale if possible.

Isolating the problem

• I can reproduce this bug in a jsbin

[kartik-v]

Fixed via #1875