Switching language part way through a recording no longer loses the second one. The speech model
picks a language per decode call, so a call holding English and then Russian committed to English
and returned nothing for the rest, with no error: the words stopped at the switch. A chunk whose
transcript ends while its speech carries on is now repacked smaller and decoded again, which is
enough for each piece to hold one language. Measured on a one-minute recording that switched half
way: 0 Cyrillic characters before, 203 after, English unchanged
Only the chunk that needs it pays for the second pass, so a single-language recording costs
nothing. The check reads one chunk on its own rather than comparing it against its neighbours,
because a one-minute recording is a single chunk and there is nothing to compare it with