Siempre que pienso en decir "más o menos" sólo pienso en "more or less" pero no estoy seguro de que sea la traducción correcta. Entonces ¿Cómo se dice en inglés?
Más o menos means at least two different things in Spanish.
When it is used as a synonym of "aproximadamente" ("approximately"), "more or less" is a fine English translation. For instance, "está a más o menos cinco kilómetros de aquí" can be translated as "it's more or less five kilometers from here."
However, when it is used to mean "no muy bien," more or less is wrong. Correct alternatives include "not too good" and "so-so". Imagine someone asked you "¿Cómo estás?" If you weren't doing too well you could indeed say "más o menos." But if someone asked "How are you?" you can't say "more or less" in idiomatic English: you can use one of alternatives mentioned above (there are plenty of other ways to say the same thing as well.)