No, it's not right. A natural, informal way to say it is "My English is rusty." A "knack" is similar to a "talent." They both refer to a natural aptitude for something, not something that is learned. A "talent" means something big and important. A "knack" means something small and limited. For example, "I don't have a real talent for cooking, but I do have a knack for making omelets." Or, "He has a knack for entertaining people at parties by playing the piano and singing, even though he doesn't have the talent to earn a living that way." Someone with "a knack for English" would mean someone who finds English easy to learn.

I think “lost the knack” for something is natural, but the first part might be better as “I haven’t spoken English in a long time”