Help me do the comma

[Alex K]

Let's eat, grandma!
Let's eat grandma!

Ummm.....

Yeah yeah.  That's my position and I'm sticking to it.

Bon, appetit

[One Above All]

But back on topic: is there an official authority on English or AE grammar where one would find these things?

http://www.mla.org
http://www.chicagomanualofstyle.org/home.html

Incidentally, for those who are curious, putting two spaces after any kind of punctuation is wrong, according to those two sources, among many, many others.

[Alex K]

Incidentally, for those who are curious, putting two spaces after any kind of punctuation is wrong, according to those two sources, among many, many others.

Two spaces?! Is that something people do?

[Pyrrho]

There are certain amusing holes in my knowledge of the language that would give it away though, such as vocabulary pertaining to activities which I rarely encounter. Names for some household appliances or tools for example.

The thing is, native speakers often do not know the right words for objects pertaining to activities which they do not normally encounter.  For example, probably most native speakers of English could not give the proper names for all of the implements used by people who climb cliffs.

Now, a name for common household appliances and common tools could expose something odd about you; if, for example, you did not know the words "toaster," "refrigerator," "stove," "oven," "hammer," "screwdriver," "wrench," "pliers," "drill," or "saw," it would seem strange.  But an unusual appliance or tool would not expose you as not being a native speaker.

[One Above All]

Incidentally, for those who are curious, putting two spaces after any kind of punctuation is wrong, according to those two sources, among many, many others.

Two spaces?! Is that something people do?

See Jörmungandr's post, for example.
EDIT: I'm not singling you out, Jörmungandr; it's just you're the first example I saw in this thread.

[Longhorn]

Incidentally, for those who are curious, putting two spaces after any kind of punctuation is wrong, according to those two sources, among many, many others.

Two spaces?! Is that something people do?

only the biggest freaks,  but you know,  it happens

[Dystopia]

As a non-native I imagine how I would use commas if I was speaking in my native language and it usually works - That being said, I was never good with commas with any language so it's not my strong part (And paragraphs are a pain in the ass).
I'm still not very good at English grammar - If someone tells me to spell a random, more complicated verb tense (like past participle) I may take a while to figure it out. I mostly played videogames and watched movies in English when I was a kid and when I had English classes in school I already knew how to write, speak and spell - I didn't know the grammar, but I could write a complex, well structured text with coherent arguments without any problem. It's kind of an oxymoron, but it's much better than being able to get the whole grammar right (it can be done using your memory) but not being able to have a decent conversation with a foreigner.

I consider my grammar decent/good but I haven't spoken English in real life for a while so my pronunciation/accent probably sucks.

[Pyrrho]

It isn't excessive.  However, it is optional, as Ben Davis stated.

I get that it is optional, I just wasn't sure whether the options are: do both 1. and 2. or neither, or whether I can pick and choose.

You can pick and choose, depending on the exact effect desired.  (I suppose I should mention, I am not an authority on English grammar.  So trust, but verify.)

...

Since it is optional, you are not wrong to leave it off.  So you are going to have to do something else to try to convince us that you are not really a native speaker of English.  Your grammar and word usage are such that one would guess you to be a well-educated native speaker.

Yes, yes, enough with the flattery already

It is not flattery.  We know you are really a native speaker of English, and are just pretending to be German.  Just like your picture; Peter Sellers was just pretending to be a German, and was really English.  That is one of the subtle clues you have provided for us.  So, what part of England are you from?

[Jackalope]

Incidentally, for those who are curious, putting two spaces after any kind of punctuation is wrong, according to those two sources, among many, many others.

Two spaces?! Is that something people do?

It's something I was trained to do when I learned to type in the early 1980's.  Two spaces after the end of any sentence.

Yeah, it's wrong.  I still do it.  Hard habit to break.

[Exian]

Words for actions and appliances can vary widely with just neighboring regions. Recently found out "sweeper" isn't normal, and of course there's always "pop". One that three me off was the question "What do you call it when it's sunny and raining." As far as I knew, there wasn't a word for it.