t() checks locale.settings.json for "translate_english" even when it does not exist

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While doing some profiling, I found that the t() function can be doing a config_get() call every single time it is called. With t() executed hundreds of times per page, this was resulting in a lot of disk accesses.

The problem is that we're checking config_get('locale.settings', 'translate_english') in t(), but the Locale module might not have been enabled in the first place. This results in locale.settings.json not existing, and the config_get() having to check for the file every request. We should optimize this by:

• Moving translate_english into system.core.json so it always exists.
• Statically caching the result of the $translate_english variable in t().

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PR filed at backdrop/backdrop#574.

This was a pretty grievous performance bug. XHProf before/after on the admin/content page:

I found that moving the translate_english option to system.core was unnecessary. We can just treat a NULL value as FALSE if Locale module is not enabled.