As part of the "Pick a sentence" step (T251551) users are expected to see only one selected sentence at a time, which can be highlighted in either yellow (if it has not been translated) or blue (if it was translated already).
However, currently as sentences get translated they keep the blue backgorund even when they are not selected:
This ticket focuses on making sure that translated sentences are only highlighted when they are selected. Other adjustments for translated sentences are captured in T260128