Стоит нам заговорить о празднике, как мама вспоминает про свечи и шарики.

Here стоит is from стоить, but not in the sense of to cost money.

In this pattern, стоить + dative + infinitive means:

• it only takes...
• one has only to...
• as soon as...

So:

• Стоит нам заговорить о празднике, как...
  = As soon as we start talking about the holiday, ... = We only have to start talking about the holiday, and ...

It is a very common Russian structure for a quick, automatic consequence.

Because this construction uses the dative case for the person involved:

• мне
• тебе
• ему
• нам
• вам
• им

So Стоит нам заговорить... literally works like:

• It only takes for us to start talking...

This is why Russian uses нам, not мы.

Compare:

• Мы заговариваем о празднике = We start talking about the holiday
• Стоит нам заговорить о празднике... = As soon as we start talking about the holiday...

The second is an impersonal pattern, so the expected form is dative.

Заговорить is a perfective verb meaning to begin speaking / to start talking.

That fits this sentence well, because the idea is that the moment the topic comes up, the next thing happens immediately.

So:

• заговорить о празднике = to start talking about the holiday

If you used говорить, the meaning would be less about the starting point and more about the process of speaking in general.

Also, the prefix за- here often gives the idea of beginning an action:

• петь = to sing

• запеть = to start singing

• говорить = to speak
• заговорить = to start speaking / start talking

So заговорить is very natural here.

This is a fixed pattern meaning:

• as soon as ... , ...
• no sooner ... than ...
• one has only to ... and ...

So the sentence structure is:

• Стоит + dative + infinitive, как + main clause

In your sentence:

• Стоит нам заговорить о празднике, как мама вспоминает про свечи и шарики.

This means that talking about the holiday immediately triggers the mother’s reaction.

Very similar alternatives are:

• Как только мы заговорим о празднике, мама вспоминает...
• Едва мы заговорим о празднике, мама вспоминает...

But Стоит нам..., как... often sounds especially like this always happens right away.

No. Here как is not the question word how.

In this sentence, как is a conjunction introducing the result that follows immediately after the first action.

So in:

• Стоит нам заговорить..., как мама вспоминает...

как is functioning more like:

• and then
• when immediately
• as soon as

This is specific to the pattern Стоит..., как...

So you should read the pair together:

• Стоит ..., как ...

rather than trying to interpret как by itself as how.

Yes, both о and про can mean about, but they are used differently.

1. заговорить о чём-то

With заговорить, the usual pattern is:

• заговорить о чём-то = to start talking about something

So:

• заговорить о празднике

is the standard choice.

2. вспоминать о чём-то / вспоминать про что-то

With вспоминать, both are possible:

• вспоминать о чём-то — a bit more neutral or bookish
• вспоминать про что-то — common, conversational

So:

• мама вспоминает про свечи и шарики

sounds natural and fairly colloquial.

A slightly more neutral version would be:

• мама вспоминает о свечах и шариках

Both are understandable; the choice is mainly about normal verb-preposition combinations and style.

Because after о meaning about, Russian normally uses the prepositional case.

So:

• праздник → dictionary form
• о празднике = about the holiday

Other examples:

• о книге = about the book
• о маме = about mom
• о друзьях = about friends

So заговорить о празднике is simply the normal case pattern.

Because про takes the accusative case.

So in:

• про свечи и шарики

the nouns are in the accusative plural.

However, for inanimate plural nouns, the accusative often looks exactly like the nominative. That is why the forms appear unchanged:

• свечи — nominative plural / accusative plural
• шарики — nominative plural / accusative plural

So even though the case is accusative after про, the forms look familiar.

Because the sentence describes a regular reaction or habitual situation.

• Every time the topic comes up, mom starts remembering or mentioning candles and balloons.

Russian often uses the present tense imperfective for this kind of repeated, typical behavior:

• мама вспоминает = mom remembers / brings up

If you used a perfective form such as вспомнит, it would point more to a single future event:

• Стоит нам заговорить о празднике, как мама вспомнит... = more like As soon as we start talking about the holiday, mom will remember...

That version is possible, but it sounds more like one specific future occasion.
The original sentence sounds like this is what she always does.

Most naturally, it means whenever or every time.

That comes from the combination of:

• the general pattern Стоит ..., как ...
• the present-tense вспоминает

So the sense is:

• Any time we start talking about the holiday, mom immediately brings up candles and balloons.

Russian often uses present tense like this for repeated, typical situations.

Yes, literally шарики comes from шар = ball / sphere, so it can mean little balls.

But in everyday context, especially with a holiday, свечи и шарики strongly suggests:

• candles and balloons

This is very natural in Russian, because balloons are thought of as round inflatable objects, so шарики is commonly used for them.

So in a party context, шарики is best understood as balloons.