Currently some locale [docs] support can only be set at initdb time, but I think the one relevant to _pattern_ops can be modified via SET at runtime, LC_COLLATE. To see the set values you can use the SHOW command.
For example:
SHOW LC_COLLATE
_pattern_ops indexes are useful in columns that use pattern matching constructs, like LIKE or regexps. You still have to make a regular index (without _pattern_ops) to do equality search on an index. So you have to take all this into consideration to see if you need such indexes on your tables.
About what locale is, it’s a set of rules about character ordering, formatting and similar things that vary from language/country to another language/country. For instance, the locale fr_CA (French in Canada) might have some different sorting rules (or way of displaying numbers and so on) than en_CA (English in Canada.). The standard “C” locale is the POSIX standards-compliant default locale. Only strict ASCII characters are valid, and the rules of ordering and formatting are mostly those of en_US (US English)
In computing, locale is a set of
parameters that defines the user’s
language, country and any special
variant preferences that the user
wants to see in their user interface.
Usually a locale identifier consists
of at least a language identifier and
a region identifier.