In French, terrible can mean both terrific and awful (or terrible). The first case (terrific) is sort of slang and would not be used in written French but is extensively (or maybe was some years ago) used by young people to say terrific. The second case (awful) is the more common and traditional one and it can be used in written French as well as in spoken French.

To differentiate them, the only clue I can give is use the context or the tone of the voice it is spoken French. A French person would probably not use terrible and say it with a cheerful tone when meaning awful since he/she is aware of the other meaning.

Finally yes, a lot a French people are mixing up the English words terrible and terrific and would in most cases use terrible to mean terrific. Please, excuse us

When I spent a year in France back in the early ’70s, everybody was using “terrible” to mean “fantastic,” “terrific,” “wonderful,” and so forth. I can testify to that, but I’m wondering if people in France still use it that way. In my frequent return visits, I would use it in the slang sense, but often wondered if people still understood it that way, or whether I was just showing my age.