Casing: Should only apply to Latin-script characters

[bwiernik]

In English locales, Greek and other non-Latin characters are used either as atomic entities (e.g., mathematical symbols) or for non-English words. In the former case, casing should not be changed. In the latter case, it's expected that words would already be stored in the correct locale-specific casing.

Accordingly, as CSL casing rules are only applied to English locales, these should only be applied to Latin-script characters.

It is currently possible with citeproc-js and other processors to override casing using <span class="nocase"> </span> tags, but requiring these tags for all mathematical symbols is onerous and seemingly unnecessary.