Conversations can be difficult to follow when you can’t hear the distinguishable accents. A difference in colour for each speaker, floating of text towards the side of the screen that the speaker is on, naming the current speaker, or even representing the speaker’s face all help – and a combination of them used together is even better.
A common effective approach is use of colour at all times, then the first time a speaker appears in a scene or any time dialogue occurs off-screen, add the the name of the speaker as text at the start of the line.
Another approach, commonly used in RPGs, is to add an indicator directly to the person speaking – for example a speech bubble above their head.
Best practice example: Portal 2
Best practice example: Tomb Raider
Best practice example: Diablo 3
Best practice example: Dream Chamber