Three properties need three checks
An audio track has actual recorded content, descriptive metadata and selection preferences. A default disposition concerns preference; it does not translate the recording or establish its language. Keep these three properties separate when producing an owned multitrack file.
Listen to a known passage in each required track. Then inspect its language and role labels. Finally review the intended default flag. Doing this in that order prevents an English label attached to a commentary track from becoming the unquestioned production record.
FFmpeg documents explicit disposition control and default behaviour in some automatic selection cases. Preserve the exact output inventory and installed-version context. Do not assume the first input audio stream and the first output audio stream always represent the same material.
Describe the handoff precisely
Imagine an owned interview package with a main mix and commentary. The main mix is correctly labelled but the commentary is marked default. A metadata correction may be appropriate; replacing the main mix or re-encoding all picture frames may be unrelated work.
After correction, inspect the output flags and repeat the intended destination's opening and manual-selection tasks. A file preference is not a promise that every destination uses identical selection rules. Record the tested route and its actual choice.
Keep a delivery sheet naming the output stream order, language and role. If the recipient requests a different starting track, update the preference deliberately while preserving every approved asset. This gives a later reviewer a clear difference between wrong content, wrong label and wrong preference.
Questions answered
Does a default flag set the language?
No. Language metadata and recorded content remain separate.
Will every player obey the same starting-track preference?
Do not assume so. Check the intended destination.
Sources & further reading
Official references for the subjects discussed. Service terms and device software can change.