Nevermind thick accents. What about languages that have tens, even hundreds, of dialects?
Slovenian, for instance, has only 2,000,000 speakers. Yet there are 56 officially classified dialects. [1] They all vary in important ways, sometimes to the point that a speaker from one dialect has trouble understanding a speaker from another.
French is another language with hundreds of dialects. [2] My girlfriend is French, but when she goes to the Caribbean, she has trouble following conversation because their French is so much different than hers.
English makes for a wonderful example too. Not only does it have hundreds of dialects, there are at least 4 different Englishes. UK, US, Aussie, African-American Vernacular. They're starting to show signs of splitting into separate languages. They already have differences in vocabulary and grammar.
Then you can add slang on top of all this.
Oh and to add to all this confusion: In most European countries people learn the standardized form of their native language almost as if it was a foreign language.
I don't think we'll ever get to 100% accuracy with a tool like this. Even humans themselves can't do 100%.
[1] https://en.wikipedia.org/wiki/Slovene_dialects#List_of_diale...
[2] https://en.wikipedia.org/wiki/Varieties_of_French