Learn Hardcore Indonesian: Saya berharap suatu hari bisa berbicara dalam konferensi internasional tentang astronomi atau robotika. - I hope one day I can speak at an international conference on astronomy or robotics.

suatu hari – already implies the future from context.
suatu hari nanti – adds nanti (“later”), making the “in the future” feeling even clearer/emphasized.

Your sentence is fine with just suatu hari; suatu hari nanti would be a slightly stronger way to say “someday in the future” but also a bit longer and more casual‑sounding.

Why is bisa before berbicara? Could I say berbicara bisa?

In Indonesian, modal verbs like bisa, dapat, harus, ingin normally come before the main verb:

bisa berbicara = “can speak”
ingin makan = “want to eat”
harus belajar = “must study”

Berbicara bisa is not natural here. You might occasionally see berbicara bisa tentang… in very specific structures, but as “can speak”, the normal order is bisa berbicara.

Why is it dalam konferensi internasional and not di konferensi internasional?

Both dalam and di can appear with konferensi, but there is a nuance:

di konferensi internasional – literally “at an international conference”; focuses on location / event.
dalam konferensi internasional – literally “in an international conference”; often feels a bit more formal and can suggest “within the context/framework of the conference”.

In your sentence, dalam konferensi internasional sounds slightly more formal and “inside the event” in a programmatic sense (as a speaker). di konferensi internasional would also be correct and very common in speech.

Does berbicara here mean just “to speak” or “to give a speech/presentation”?

Literally, berbicara = “to speak / to talk”.
In context with konferensi internasional, it is naturally interpreted as:

“to speak (as a speaker at the conference)”
“to give a talk / presentation”

So bisa berbicara dalam konferensi internasional is like saying “be able to speak at an international conference”, implying being one of the speakers, not just casually talking to people.

Why is saya only at the beginning? Could I say Saya berharap saya suatu hari bisa…?

Indonesian often drops repeated subjects when it’s clear from context.

Saya berharap suatu hari bisa berbicara… – The subject of bisa berbicara is still understood to be saya, so it doesn’t need to be repeated.
Saya berharap saya suatu hari bisa berbicara… – Grammatically possible, but sounds a bit redundant or heavier in everyday speech.

Usually, the shorter original version sounds more natural and smooth.

Why is it konferensi internasional, not internasional konferensi?

In Indonesian, descriptive words (often called “adjectives”) usually come after the noun:

konferensi internasional = “international conference”
buku baru = “new book”
mobil merah = “red car”

Putting internasional before konferensi (internasional konferensi) is ungrammatical in standard Indonesian.

What is the function of tentang in tentang astronomi atau robotika? Could I use mengenai instead?

Tentang means “about / regarding”:

tentang astronomi atau robotika = “about astronomy or robotics”

You can often replace it with mengenai, which also means “about, regarding”:

berbicara dalam konferensi internasional mengenai astronomi atau robotika

Differences:

tentang – very common, neutral, works in speech and writing.
mengenai – slightly more formal or written style, but also used in speech.

Both are correct here; tentang is a perfectly natural choice.

Why is it astronomi atau robotika and not astronomi dan robotika?

atau = “or”
dan = “and”

So:

tentang astronomi atau robotika = “about astronomy or robotics (one or the other)”
tentang astronomi dan robotika = “about astronomy and robotics (both)”

The original sentence expresses a wish to speak about either astronomy or robotics; changing to dan would slightly change the meaning.

Is there any difference between bisa and dapat here? Could I say dapat berbicara?

Yes, you can say dapat berbicara instead of bisa berbicara:

bisa berbicara – “can / be able to speak”; very common in everyday speech.
dapat berbicara – also “can / be able to speak”; often feels a bit more formal or written, though people also use it in speech.

In a sentence about future aspirations, both work:

Saya berharap suatu hari bisa berbicara…
Saya berharap suatu hari dapat berbicara…

The meaning is essentially the same.

How formal is Saya berharap…? Could I use semoga instead?

Saya berharap… = “I hope (that)…” and is neutral to formal. Very acceptable in:

writing (essays, emails, articles)
speeches, interviews
polite conversation

Semoga is another way to express hope, like “hopefully / may (it be that)…”:

Semoga suatu hari saya bisa berbicara dalam konferensi internasional…

Nuance:

Saya berharap… focuses on you as the hoper.
Semoga… sounds a bit more impersonal or wishful, like a general hope or blessing.

Both are correct; your original version is slightly more explicit and formal.

Why don’t we have sebuah before konferensi? Should it be sebuah konferensi internasional?

Indonesian does not require an article like “a” or “an” as often as English.

konferensi internasional can mean “an international conference” or “the international conference” depending on context.
sebuah konferensi internasional explicitly marks it as “one / a certain international conference”.

Your sentence is natural without sebuah. Adding sebuah slightly emphasizes “one particular (unspecified) conference”, but it’s not necessary:

…bisa berbicara dalam konferensi internasional… ✅
…bisa berbicara dalam sebuah konferensi internasional… ✅ (a bit more specific)

Is it okay to drop Saya and just say Berharap suatu hari bisa berbicara…?

In very casual writing (social media, chats) people sometimes drop Saya:

Berharap suatu hari bisa berbicara…

But strictly speaking, this is more like a fragment. For clear, standard Indonesian—especially in anything semi‑formal—it’s better to keep Saya:

Saya berharap suatu hari bisa berbicara dalam konferensi internasional…

So: acceptable in informal contexts, but Saya is recommended in proper sentences.