I stuttered for 65 years and at 70 I became fully fluent through dealing with my abusive childhood in psychotherapy. Though I’ve been totally fluent in English, my native language, when I speak Spanish( I grew up in South Texas and I had to speak Spanish to survive) but when I’m conversing in Spanish I often slightly stutter. I think it’s like OP related, the added stress of remembering, translating in your head and guarding against mistakes, causes just enough stress to initiate stuttering, even though I no longer stutter when speaking English. Sue