I have in my notes that the format по-...-ски is required over на ...-ком succeeding the verbs - говарить, читать, писать, понимать & думать.
This rule is discussed somewhat here.
However, using a search engine I found that there are varying distributions of how these are actually used. Below is a list, with the Russian form, the English translation and in brackets the percentage of that form used as returned by the search engine.
Я не говорю по-русски. - I don't speak in Russian. (70%)
This means 70% of instances were for [говорю по-русски] and 30% for [говорю на русском].
- Я не говорю по-русски. - I don't speak in Russian. (70%)
- Я скажу по-русски. - I will speak in Russian. (65%)
- Я только понимаю по-русски. - I only understand Russian. (70%)
- Я пойму по-армянски. - I will understand Armenian. (20%)
- Я читал по-английски. - I read in English. (50%)
- Я прочитаю по-испански. - I will read in English. (20%)
- Я пишу по-немецки. - I write in German. (30%)
- Я напишу по-норвежски. - I will write in Norwegian. (30%)
- Я всегда думаю по-итальянски. - I always think in Italian. (30%)
It seems the forms that should be used are "говорить", "сказать", "понимать" and, maybe, "читать". Is this correct? And what could be said of people who are using the forms like говорю на русском? Is this a mistake? A common error, or are there circumstances in which people could use говорю на русском?