Both selects grow a "Frequently used" optgroup above "All languages",
holding up to ten entries ranked by how often each language has actually
been translated to or from.
Counted on a successful translation, not on a dropdown change: picking
your way down the list looking for something would otherwise rank every
language you skimmed past as highly as the ones you work in. A
detected source counts too — with Auto-detect on you never pick that
language explicitly, but it is one you read.
Ties break on the language's display name so the order is stable rather
than dependent on object key order. Counts live in the existing uiState,
so they persist through the saveUi/loadState path already there, and the
group only appears once there is something to put in it.
Verified in the harness: one translation records exactly one use for the
target and one for the detected source; the cap holds at ten with
fifteen tracked; and a pre-seeded profile comes back with its group,
target and zoom restored, Auto-detect still first in the source select.
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