Improve language list regarding dialects

[lcjohnso]

Current Dialect-related Issues:

1. Some language choices are only provided with dialects included (bn-in and bn-bd, but no plain bn), but we have a preference for locales w/o dialects. Plain options should be added where applicable and reasonable.
2. Some language choices have one or more dialect options available, however we typically (except for Chinese) do not support locales with dialect specifications, so these options should be removed (e.g., en-us and en-gb).

Finally: the default long language lists (i.e., constants/languages.js) should be kept in sync between PFE and Pandora. Some differences currently exist, and any dialect-related changes should be made to both apps.