printing severity level as string in log file

You may show us how your current log function is defined and how you print. Without seeing this, it is difficult to answer.

If the severity is defined by an integer, you usually would have an array of severity strings:
const char lpszSeverities[] = { "Emergency", "Alert", /*..., */ "Debug" };
fprintf(fOut, "%s: %s", lpszSeverities[nSeverity], lpszMsg);