Most people who speak English in South Africa are bilingual in Afrikaans. This can be as simple as Brits ordering chips while North Americans order French fries.
Conservative use of all these techniques will give them dignity and authenticity. Whatever combination of techniques you use to render an accent or dialect in dialogue, use them with a light hand.
So what can you do to avoid falling into this trap with your foreign-accented characters or those who speak in a pronounced dialect? These individuals were largely "standard speakers" like retired military personnel and aristocrats.
Use a combination of all the techniques described here. Even standard English is a dialect. Occasionally, you can leave the dialogue in standard English and simply describe the accent to readers.