LanguageConverter performance: Reuse the same string object for regexp

The regular expression used by LanguageConverter::autoConvert() is a
constant, but it is being created on-the-fly by every invocation.
This causes an expensive full-string comparison when the compiled
regular expression is fetched from the cache -- since the regex is 332
bytes long, the time taken for this comparison can add up quickly: on
page with a lot of tags, the regexp cache may spend more time looking
up the regexp than it takes to execute it.

Bug: T223969
Change-Id: I53c3e631e47a791cf3f0844dd79d4357605c59e3