Language examples: Romanian, Latvian and Livonian use comma, rather than cedilla (as used in French and Turkish on C, c, and in Turkish with S, s).
Rule of thumb: use C with cedilla and G, K, L, N, R with comma. To use S and T correctly you have to know the language of the text.
Note that in practice, in some cases cedillas may appear below Romanian letters, and in some cases commas may appear below Turkish letters, but Romanians now generally prefer commas and Turks usually prefer cedillas below. Care should be taken to ensure predictable coding, even if printing and display may differ from what is coded.