La lando estas en la unio.

Because both lando and unio are definite nouns here.

• la lando = the country
• la unio = the union

In Esperanto, la means the, and you normally repeat it before each noun that needs it. So La lando estas en la unio is the normal pattern.

The ending -o marks a noun in Esperanto.

So:

• lando = country
• unio = union

This is one of the most basic Esperanto patterns:

• nouns end in -o
• adjectives end in -a
• adverbs often end in -e

Because both lando and unio are nouns, they both end in -o.

Estas is the present-tense form of esti, which means to be.

So:

• estas = is / am / are

A very important point for English speakers: Esperanto verbs do not change according to the subject.

For example:

• mi estas = I am
• vi estas = you are
• la lando estas = the country is
• ili estas = they are

So estas stays the same no matter who or what the subject is.

En usually means in or inside, and here it expresses being within a group or organization.

So en la unio means in the union.

This does not have to mean physically inside something. It can also mean membership or inclusion. In this sentence, it most likely means the country belongs to the union.

Because this sentence describes location or membership, not movement toward something.

• en la unio = in the union
• no -n is needed

In Esperanto, -n is used for the direct object, and sometimes after a preposition to show direction or movement.

Compare:

• La lando estas en la unio. = The country is in the union.
• Li iras en la domon. = He goes into the house.

In the second example, domon has -n because there is movement into the house. In your sentence, there is no movement, so unio stays as unio.

Grammatically, you can omit la when you do not mean the, but that would change the meaning and usually sound strange here.

• La lando estas en la unio. = The country is in the union.
• Lando estas en unio. = A country is in a union

The second version is much more general and less natural in most contexts. If you are talking about a specific country and a specific union, you need both instances of la.

Both can sometimes relate to country, but they are not exactly the same.

• lando = country, land
• ŝtato = state, political state

Lando is often the most natural translation of everyday English country.
Ŝtato is more political or governmental.

So in a simple sentence like this, lando is very natural.

Yes, Esperanto word order is fairly flexible, but La lando estas en la unio is the most neutral and straightforward order.

This is the basic pattern:

• subject: La lando
• verb: estas
• prepositional phrase: en la unio

You could move parts around for emphasis, but that may sound more marked or literary. For a learner, the standard order is the best choice.

In this sentence, lowercase unio is correct if you mean a union in a general sense.

If you mean a specific proper name, such as the European Union, Esperanto normally capitalizes the proper-name part:

• La lando estas en la Eŭropa Unio.

So:

• la unio = the union, generic
• la Eŭropa Unio = the European Union, proper name

It can mean membership too.

English speakers sometimes think in only means physical position, but in Esperanto, en can also describe being part of a group, institution, or system.

So here, La lando estas en la unio is naturally understood as something like:

• the country is part of the union
• the country belongs to the union

The exact nuance depends on context, but the grammar is completely normal.