When searching for site texts for admin using the english
version of the text, previously we would show the english
version in the results _even if_ there was another locale
translated version available when a locale was selected
from the dropdown.
This commit adds a "Only show results in selected locale"
checkbox option which will instead make it so the results
shown are in the target locale, making it easier for translators
to tell when there is actually translations vs. missing tranlsations.
