Please move from eng/fra file name suffixes to en/fr

[pjackson28]

Now that we are approaching WET v4.0, we will be aligning with i18n conventions and changing our file name suffixes from eng/fra to en/fr. If you use eng/fra suffixes for any of your files then please migrate to the en/fr suffixes at your convenience.

To ensure you don't disrupt search engine results, please use either 301 redirects or canonical rels to help search engines transiton from providing results with eng/fra to en/fr (WET core is using canonical rels because GitHub gh-pages doesn't support 301 redirects).