First, upgrade if at all possible. Yosemite is probably the worst OS 10 release. Latest version of El Capitan isn’t bad, and Sierra is almost a good OS—stable and predictable, at any rate.
Where did you DL Gimp? DL only from GIMP.org <http://gimp.org/>, English version. If that looks normal, the problem is not your OS, though see below.
As I don’t use the program in a non-English version on a non-English computer, I’m not sure how to diagnose the problem if your OS is running in French (from your sign-off, I’m assuming French. Apologies if I’m wrong.
Also, go to System Preferences, Language and Region, and see what is set there. Try setting the region temporarily to somewhere English speaking.