I agree that it would be nice if language was part of a message.

If you have your logs set to Info level or higher, you can search them for "detect language" to see if anything is being detected. Those messages come from the detect_language() function in include\language.php.

Example:
2026-09-20T23:05:41Z:LOG_INFO:32dc13c8b6:language.php:345:detect_language: detect language: Array
(
    [en] => 0.42381845461365
)