(dev/core#1546) Fix translation of seed data

(dev/core#1546) Fix translation of seed data

Overview
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This fixes a regression in which seed data is not translated.

Before
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The `civicrm_data.*.mysql` files are not translated.

After
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The `civicrm_data.*.mysql` files are translated.

There's a unit-test to check this.

Comments
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This regression stems from #15411, which aimed to allow extensions to define
custom variants of `ts()`.  The crux of the issue is "What happens if you
try to translate a string before the system is bootstrapped - before the
extension is loaded?  What's your fallback behavior?"

In #15411, it used a fallback behavior of "do no translation".  In theory,
you shouldn't really get into this scenario since UIs are pretty much always
generated post-boot.

However, it turns out that there is a situation where you have an un-booted
system and need to translate strings -- i.e. when generating the localized
`civicrm_data.*.mysql` data.  Hence the bug.

This patch preserves most of the changes from #15411, but it changes the
fallback behavior from "do no translation" to "use the built-in/default
translator".