Provide language specific template files

[simonwheatley]

Allow theme developers to append the language code to any given WordPress template file, and have that override the non-language specific version. For example:

• index-fr-fr.php overrides index.php for French
• taxonomy-movie-the-shining-pt-br.php overrides taxonomy-movie-the-shining.php for Brazilian Portuguese
• (etc)

[simonwheatley]

See the various comments here for some context and discussion: https://github.com/cftp/babble/pull/119#issuecomment-24529966

Re Core Ticket #14310: We have had a brief conversation with Westi, who suggests that we should code using existing filters in order in the first instance, if this proves a nightmare we can then approach core with a specific real world issue.