Sometimes, I find myself in a conversation with people who can't speak English (or some other language) with standard pronunciation. Of course, not everyone has good speaking skills to begin with, and I believe it's more of an issue with my hearing than everything else. Moreover, I don't want to discourage anyone from learning a new language by making remarks about their skills.
However, when I have a decent understanding of the interlocutor's native language, I feel like the conversation evolves more naturally if they use it.
For instance, I've just helped a French tourist get around my hometown. While I couldn't understand her strongly accented English, the conversation went smoothly as soon as she started speaking French to me. I can't speak French, so I just answered her questions in English, making it a bilingual conversation. Due to the circumstances - we were at the subway, plus she's a stranger I'm unlikely to ever see again -, I was able to say "I'm sorry, but I cannot understand your English, can we try French, please?", but this sounds like a rude way to start talking to people I must see on a daily basis.
In another example, one of my best friends' wife is Peruvian and I struggle to have a talk with her in any language other than Spanish. Having been living here for several years, she has a decent vocabulary and most people get along with her just fine (in fact, she's lovely). Yet, whenever we're talking using the local language, I seem to miss about 20% of what she's saying and I don't have the courage to tell her.
How can I politely get someone to talk to me in an understandable manner, without making them feel like it's their fault somehow?
I'm ok with little white lies, as long as it doesn't discourage them from keeping on trying. I do feel like this is more of an issue with me than the other person, but saying this out loud sounds like I'm just trying to call them out politely.