Feeds integration: support mapping option "language"

Problem/Motivation

With Feeds now officially supporting field translation using Entity translation, mapping targets provided by contrib modules should be adjusted to follow this change.
For fields that can be multilingual, Feeds added a mapping option "language" which is passed to all target callbacks. To support Feeds 7.x-2.0-beta2 and later, this option should be respected.

Proposed resolution

Follow the instructions in the change record: use $mapping['language'] at places where LANGUAGE_NONE or und is used.

Remaining tasks

Apply the proposed resolution.

User interface changes

When the field is configured as multilingual, on the target configuration an option will become available to configure the language. This module is expected to respect that option.

API changes

None.

Data model changes

None.

A patch will follow.