"as Béarla" is as standard as "i mBéarla". I don't know why they tend to teach more "as Béarla" than "i mBéarla", since most people say only "i mBéarla". But that's once again one of the mysteries of the CO.
According to what I read, in Munster Irish, people use "as Béarla" (as Gaelainn, etc) when talking about the spoken language, and "i mBéarla" (i nGaelainn...) when talking about the written language.
Do bhí sé 'caint as Gaelainn
Do bhí 'n leabhar seo scríofa i nGaelainn
I don't know if that rule is true in whole Munster.
Anyway, looks like 100% of the native Irish speakers use "i", and (some?) native speakers from Munster also use "as" when talking about the spoken language. So... what should be taught should be "i".
Bhí mé ag caint as Béarla leis
Ulster: Bhí mé 'labhairt i nGaeilic leis (I was speaking in Irish with him)
you can also say
Bhí mé 'labhairt Gaeilice leis (I was speaking Irish with him)