I often meet people who do not natively speak my language (German) but are learning it and would love to practice it by speaking German to me.
The problem is that I talk in a slight dialect (Norddeutsch), speak very fast and often kind of unclear. Even other Germans occasionally have a hard time understanding me. I usually say just that - "I'm sorry, but [insert what I just said]". They usually suggest to try it anyways, I agree and as soon as I realize they don't understand my normal German, I switch to a forcibly slow, clear and accent free speech - often still with no success.
This is incredibly excruciating for me, not at least because I'm insecure about the whole not being understood thing in general. (Especially my dad treats it very rudely, I might tell him and his girlfriend a story and when I'm finished he'd just look at her and ask "Didya get any of that?" which makes me very self-conscious about this, but that's a whole different story.)
When I then again suggest conversing in English, the other person usually responds with "Oh no, it's fine, I can do it!" and I'm just thinking "Yeah but I can't".
In one extreme case, I had an online friend who was learning German and he got so angry with me always giving up speaking my native language that it contributed to the end of the friendship. Apparently, even if I say that the problem is on my side and my insecurities it's still taken in offence.
The other times I have these situations with people, besides online, would be while doing sports. I do Parkour, where often international people come to check it out - they might be refugees that are just learning German or transfer students from literally anywhere. Besides conversation content, it usually goes as described above.
How do I politely tell someone that I do not want to practice my native language with them?