One of my sisters taught German at a private school for decades. She is a fluent speaker of German and knowledgeable about the minutiae of German grammar. In retirement, and particularly during the COVID-19 lockdown, she has been trading German lessons for Spanish lessons with another retired colleague who taught Spanish. They meet on Zoom to speak, and give one another practice exercises and options for exploration on DuoLingo. I admire her for even taking on this enterprise.
What is interesting is that despite being an expert at one foreign language, and despite having had some Spanish in high school, she often complains that she just can’t seem to remember all of the new vocabulary she is supposed to be learning. Not long ago she remarked that she has a new sympathy with older immigrants trying to learn English and struggling. Ahhh.