Yes, I overcame my stuttering at the age of 70 through psychotherapy dealing with an abusive childhood. I've been fluent in English for 6 years, it's my native language, however, when speaking Spanish, I mildly stutter. I think as you do, it's the additional stress of having to translate in my head and remember what I'm going to say in Spanish, combined with the more limited vocabulary than English. Because I no longer use any of the techniques to avoid stuttering in English, I'm not looking to substitute words in Spanish. It must be the slightly additional stress of not speaking in my mother tongue, English.