Ninapozungumza na marafiki, upweke wangu hupungua mara moja.

It breaks down like this:

• ni- = I
• -na- = present/general time marker
• -po- = a relative marker meaning when / at the time when
• -zungumza = speak / talk

So ninapozungumza means when I speak or when I am speaking.

It sounds like a general or repeated situation: whenever I speak with friends, my loneliness decreases right away.

Two things help create that feeling:

• ninapo-... = when / whenever I...
• hu- in hupungua = habitual or regular action

So this is not just one event; it describes something that tends to happen.

Here na means with.

The verb kuzungumza is commonly used with na for the person you are speaking with:

• kuzungumza na rafiki = to speak with a friend
• kuzungumza na marafiki = to speak with friends

Remember that na can also mean and, so the meaning depends on context.

Marafiki means friends.

The singular is rafiki = friend.

So:

• rafiki = friend
• marafiki = friends

Upweke means loneliness, solitude, or being alone, depending on context.

Wangu means my, so:

• upweke wangu = my loneliness

In this sentence, upweke wangu is the thing that is decreasing.

Because possessives in Swahili agree with the noun class of the noun, not just with the English meaning.

Upweke is an abstract noun with u-, and its possessive agreement here is wa-, giving:

• wangu = my

So upweke wangu is correct.

This is a very common thing for English speakers to notice: Swahili possessives change form depending on the noun they go with.

Hupungua uses hu-, which gives a habitual or general meaning: it decreases / it tends to decrease.

That fits this sentence well, because the idea is:

Whenever I speak with friends, my loneliness decreases.

If you said unapungua, it would sound more like a non-habitual present meaning such as is decreasing or decreases in a more immediate, specific situation.

Also, if you were using ordinary present agreement instead of hu-, upweke would take u-, so unapungua would be the expected form, not inapungua.

Kupungua means to decrease, to become less, or to be reduced.

So upweke wangu hupungua literally means something like:

my loneliness becomes less

That is a very natural way to express the idea in Swahili.

It can mean either, depending on context.

Literally, mara moja is one time / once, but very often it means immediately, at once, or right away.

In this sentence, the meaning is clearly immediately / right away:

my loneliness decreases right away

Yes. Kuongea and kuzungumza can both mean to speak / to talk.

So Ninapoongea na marafiki... would also sound natural.

A rough difference is that kuzungumza can sound a little more neutral or slightly more formal, while kuongea is very common in everyday speech.

Yes. Swahili allows some flexibility.

The original sentence starts with the when clause:

Ninapozungumza na marafiki, upweke wangu hupungua mara moja.

You could also say:

Upweke wangu hupungua mara moja ninapozungumza na marafiki.

Both are understandable. Starting with ninapozungumza na marafiki puts the situation first, which is often a natural way to set up the main statement.

The comma is helpful because the sentence begins with a time clause:

Ninapozungumza na marafiki, ...

It works much like English punctuation after an opening clause such as When I speak with friends, ...

In informal writing, some people may leave it out, but using it here is clear and natural.