There is a world of difference between supposedly not speaking your mother tongue 'properly' and not havigna mastery of a foriegn language, a world of difference.Heard some of the white youth today? Most English people can't even speak English properly these days either!
The kind of errors a foreigner makes speaking a language are quite different to those made by mother tongue speakers, the latter being simply those features of grammar which within the milieu the speaker moves are in fact a 'standard' form of English and perfectly well understood by everyone around them, even if they do not conform to the accent/dialect of speech accepted by the middle classes.
These attitudes towards working class speech have always been (snobbishly) effected by the middle and upper classes, even towards what are now seen as rather charming (though not so charming that they are consciously emulated) traditional dialects such as Cockney, Geordie, or Scouse. Nowadays the snobbish remarks of the above poster tend to be reserved for those who speak teh evolved unattractive dialect influenced by whatever is the local dialect stirred in with Cockney (due to TV and internal migration) and Caribbean English (also due to TV influences and migration).
Ugly and unbgrammatic as it seems however it is not a variety of English, and not a barrier to the understanding if formal English whicha poor knowledge of our language is in an immigrant for whom English isnt a mother tongue.