Learn Hardcore Indonesian: Dialah yang menjadi perwakilan tim hari ini. - He/She is the one who becomes the team representative today.

-lah adds emphasis/focus to the word it attaches to. Dialah highlights that the person in question (he/she) is the one, often with a contrastive feel: “It’s he/she (and not someone else).” The sentence works without -lah; it just sounds less emphatic.

Can I just say Dia yang menjadi perwakilan tim hari ini?

Yes. Dia yang… is very common and natural, especially in speech. Dialah yang… simply adds stronger focus to dia.

What is yang doing here?

Yang introduces a clause that specifies or identifies the noun before it. Here, yang menjadi perwakilan tim hari ini narrows down who dia is: “the one who becomes/serves as the team representative today.” It’s like an English cleft: “It is he/she who…”

Can I drop yang and say Dialah menjadi perwakilan tim hari ini?

That sounds odd. If you drop yang, use either:

Dialah perwakilan tim hari ini (a simple identification), or
Dia menjadi perwakilan tim hari ini (no focus/cleft).

Why use menjadi here? Could I use adalah or sebagai instead?

menjadi = “become/serve as,” often implying taking on a role (even temporarily): Dia menjadi perwakilan…
adalah = copula linking two nouns: Dia adalah perwakilan tim hari ini (stating identity).
sebagai = “as,” but it usually needs a verb: Dia bertindak/berperan sebagai perwakilan tim hari ini. Plain Dia sebagai perwakilan… is incomplete as a standalone sentence.

Could I say mewakili instead of menjadi perwakilan?

Yes. mewakili means “to represent,” and it’s often more concise: Dialah yang mewakili tim hari ini or Dia mewakili tim hari ini. The meaning is essentially the same here.

What’s the difference between perwakilan and wakil?

Both can mean “representative,” but:

wakil most directly means “a deputy/representative (person),” so wakil tim often sounds very natural.
perwakilan can mean “representative (person), representation (abstract),” or “representative office.” In this sentence, perwakilan for “representative (person)” is fine, but many speakers would also say wakil tim.

Do I need dari? Is perwakilan tim different from perwakilan dari tim?

Both are acceptable. perwakilan tim is a tighter noun-noun compound; perwakilan dari tim is a bit more explicit and conversational. There’s no real change in meaning.

Does hari ini here mean “for today”? Where can I put it?

Yes—practically it means “for today.” You can place it at the end or at the beginning:

Dialah yang menjadi perwakilan tim hari ini.
Hari ini, dialah yang menjadi perwakilan tim. Adding untuk makes the “for today” nuance explicit: … untuk hari ini.

How do I say this in the past or future?

Past: Dialah yang menjadi perwakilan tim kemarin. / Dia yang mewakili tim tadi.
Completed aspect: Dia sudah menjadi perwakilan tim.
Future: Dialah yang akan menjadi perwakilan tim besok.

Can I use ia, beliau, or ialah instead of dia?

ia is a more literary/neutral subject pronoun. In speech, dia is more common. You can say Ialah yang… in some formal styles, but in Indonesian it functions more like a copula and is less natural than Dialah yang… for focusing a person.
beliau is a respectful third-person singular for elders/superiors: Beliaulah yang menjadi perwakilan tim hari ini.

Is it spelled Dialah or Dia-lah?

Write it as one word: dialah. The particle -lah attaches to the preceding word without a hyphen.

How would this sound in casual vs formal Indonesian?

Casual: Dia yang jadi wakil tim hari ini.
Neutral: Dia yang mewakili tim hari ini.
Formal/emphatic: Beliaulah yang menjadi perwakilan tim pada hari ini.

Can I rephrase to focus on the team instead of the person?

Yes, use a passive with di-:

Tim itu diwakili oleh dia hari ini. More formal: Tim itu diwakili olehnya hari ini.

Is there any number (singular/plural) ambiguity with perwakilan?

Indonesian doesn’t mark plural on nouns. perwakilan here is understood as singular from context. To be explicit, you can say seorang perwakilan or seorang wakil.