Provide language specific template files

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)

Start by locating the WordPress theme template resolution entry point and reviewing how non-language-specific template files are selected. Use the examples in the issue to trace the expected precedence, and consider the work done when language-suffixed files override their base templates for the listed locale patterns.

Written by the indexing model from the issue text.