Mi amiga habla japonés.

Because amiga is the feminine form of amigo. It tells you the friend is female.

• amigo = male friend
• amiga = female friend

The possessive mi stays the same for both:

• mi amigo = my male friend
• mi amiga = my female friend

These are two different words.

• mi (no accent) = my
• mí (with an accent) = me, used after prepositions

So here you need mi amiga because it means my friend.

Compare:

• Mi amiga habla japonés. = My friend speaks Japanese.
• Ella habla de mí. = She talks about me.

Habla is the third-person singular form of the verb hablar in the present tense.

The verb changes depending on who is doing the action:

• yo hablo = I speak
• tú hablas = you speak
• él / ella / usted habla = he / she / you speak
• nosotros hablamos = we speak
• vosotros habláis = you all speak
• ellos / ellas / ustedes hablan = they / you all speak

Since mi amiga = she, the correct form is habla.

Spanish often leaves out subject pronouns because the verb ending already shows who the subject is.

So instead of saying:

• Ella habla japonés.

you can just say:

• Habla japonés.

In your sentence, the subject is already clearly stated as mi amiga, so adding ella would usually be unnecessary:

• Mi amiga habla japonés. = natural
• Ella mi amiga habla japonés. = not correct in normal Spanish

You could say Mi amiga, ella habla japonés, but that sounds emphatic or marked, not neutral.

In Spanish, names of languages and nationalities are normally written in lowercase.

So:

• inglés
• español
• francés
• japonés

This is different from English.

So Mi amiga habla japonés. is correct, not Japonés.

The accent mark shows where the stress goes: ja-po-NÉS.

Without the accent, Spanish stress rules would suggest a different pronunciation. The written accent tells you the last syllable is stressed.

It is also just the correct spelling of the word:

• japonés = Japanese (singular masculine / the language)
• japonesa = Japanese woman / Japanese feminine adjective
• japoneses = Japanese people / masculine plural
• japonesas = feminine plural

Here japonés refers to the language.

After verbs like hablar, estudiar, aprender, and saber, Spanish usually does not use the article before a language.

So you normally say:

• Habla japonés.
• Estudia español.
• Aprendo francés.

Using el here would usually sound unnatural in a basic sentence.

It can mean both, depending on context.

In this sentence, after habla, it clearly means the Japanese language:

• Mi amiga habla japonés. = My friend speaks Japanese.

But in another sentence, it could refer to a person or adjective:

• Es japonés. = He is Japanese.
• Un estudiante japonés = a Japanese student

So the verb and context tell you the meaning.

Usually, to say someone knows or speaks a language, Spanish uses hablar + language with no preposition:

• Mi amiga habla japonés.

Hablar en japonés is possible, but it usually emphasizes the language being used in a particular situation:

• En clase, mi amiga habla en japonés. = In class, my friend speaks in Japanese.

So for the general ability she speaks Japanese, the most natural sentence is:

• Mi amiga habla japonés.

The normal word order is:

subject + verb + object/complement

So:

• Mi amiga = subject
• habla = verb
• japonés = complement

Spanish word order is often flexible, but this is the most neutral and common order.

You might hear different orders for emphasis, but for a learner, Mi amiga habla japonés is the best basic pattern to follow.

A careful pronunciation would be roughly:

mee ah-MEE-gah AH-blah hah-poh-NESS

A few useful notes:

• mi sounds like mee
• amiga stresses the second syllable: a-MI-ga
• h in habla is silent
• j in japonés is a strong throaty sound in Spain, like the ch in Scottish loch
• japonés stresses the last syllable because of the accent mark

So:

• Mi amiga habla japonés
• mee ah-MEE-gah AH-blah hah-poh-NESS

Normally, mi amiga means my female friend, not my girlfriend.

For girlfriend, Spanish usually says:

• mi novia

So:

• mi amiga = my female friend
• mi novia = my girlfriend

Sometimes context can create ambiguity, but in most cases mi amiga is just my friend.