"Is your diet effective?"
Translation:Είναι αποδοτική η δίαιτά σου;
Not exactly but they are often used interchangeably in Greek.
In English, we would use "efficient" for "αποδοτικός" and "effective" for "αποτελεσματικός".
The difference in English is that "efficient" would refer to the way something is done "He is an efficient workman and does things with the least waste of time and effort. etc." whereas, "effective" refers to the end result.
This is the way I learned it: "Being efficient is about doing things right while being effective is about achieving the right result."
Here's another example: "Walking may be an effective way to get to the office, but driving is more efficient." Both methods will get you there, but driving takes less time and energy.
Hope this helps.