Standards of Written English

I have just finished grading some first-year written assignments, and as I was doing this, I was wondering how much of the material I should be marking as incorrect.

Many people nowadays believe that there should be substantial latitude for the way people speak: there is no need for someone to pretend to come from the UK if they don't come from the UK, so it is fine to sound Bruneian or Singaporean just so long as you remain easily intelligible. But what about writing? How much should I allow my students to develop a local style of writing?

I often come across sentences like:

My sister, she speaks Malay at home.
My father, he uses only Brunei Malay to talk with us.

I feel that these are not well written, as it would be better to say:

My sister speaks English at home.
My father uses only Brunei Malay to talk with us.

However, placing the topic prominently out at the front of the sentence seems to be a feature of local English, not just in Brunei but in Singapore as well. So should I oppose it? Am I imposing an inner-circle bias against local norms?

Another sentence I constantly see is:

A research was carried out.

For me, this is ungrammatical, as research is a non-count noun, so it cannot be preceded by the indefinite article. But am I fighting a losing battle? Is research becoming a count noun, in this part of the world at least, to join furniture, advice, lighting and many more?

Finally, I tend to correct sentences such as this:

I stayed in Maura when I was young.

For me, stay is for a short time (like a couple of weeks), while we use live for longer periods of residence. However, in Brunei and also Singapore, stay and live seem to be synonyms.

Note that these represent three different areas: the use of prominent topic fronting is discourse; the issue with count and non-count nouns involves grammar; and the use of stay or live is lexical.

So, should I be correcting any of them? My feeling is that I should, as only by doing that will I help my students to improve their English and thereby enhance their future prospects. But I admit that I am being hypocritical, as in much of my work I insist on being descriptive, not prescriptive; and, in theory at least, I support the emergence of regional varieties of English.

I have no easy answer to this question.