Mi madre es realista: sabe que aprender un idioma lleva tiempo.

Because realista describes a more permanent quality or outlook, not a temporary state.

• ser + adjective is used for characteristics: es realista = she is realistic / a realist
• estar + adjective is usually for states or conditions: está cansada, está contenta

So here, the sentence is talking about the mother’s general way of thinking, which is why es is correct.

Here, realista means realistic or a realist.

In this sentence, it means the mother sees things as they really are and does not expect learning a language to happen quickly.

A useful detail: realista has the same form for masculine and feminine.

• Mi padre es realista
• Mi madre es realista

The colon introduces an explanation of the first idea.

So the structure is:

• Mi madre es realista:
  explanation follows
• sabe que aprender un idioma lleva tiempo.

It works a lot like English:

• My mother is realistic: she knows that learning a language takes time.

A comma could sound weaker, while the colon neatly signals here is the reason / explanation.

This is a very common and important distinction.

• saber que + clause = to know that...
• saber + infinitive = to know how to do something

So:

• sabe que aprender un idioma lleva tiempo = she knows that learning a language takes time
• sabe hablar español = she knows how to speak Spanish

In your sentence, the meaning is she knows that..., so que is necessary.

Because in Spanish, an infinitive can act like a noun and be the subject of a sentence.

Here, aprender un idioma means learning a language.

So the structure is:

• aprender un idioma = subject
• lleva tiempo = verb + complement

This is very natural in Spanish. English does something similar with -ing:

• Learning a language takes time.

Spanish often uses the infinitive where English uses -ing.

Because the whole infinitive phrase aprender un idioma is treated as one singular idea.

So:

• Aprender un idioma lleva tiempo.
• Learning a language takes time.

Even though the phrase contains several words, grammatically it functions as one subject, so the verb is singular: lleva.

It means takes time.

Literally, llevar often means to carry or to take, but in this kind of sentence it is used idiomatically:

• Eso lleva tiempo = That takes time
• Aprender un idioma lleva tiempo = Learning a language takes time

This is very natural Spanish. In Spain, llevar tiempo sounds especially idiomatic in this meaning.

Because Spanish often uses nouns like tiempo, dinero, or paciencia without an article when speaking in a general sense.

So:

• lleva tiempo = takes time
• cuesta dinero = costs money
• requiere paciencia = requires patience

If you added an article, the meaning would become more specific:

• lleva tiempo = it takes time, generally
• lleva mucho tiempo = it takes a lot of time
• lleva el tiempo necesario = it takes the necessary time

In your sentence, the general idea is intended, so no article is needed.

Because idioma is a masculine noun, even though it ends in -a.

So it is:

• un idioma
• el idioma

not:

• una idioma
• la idioma

This happens with some words of Greek origin, such as:

• el problema
• el sistema
• el tema
• el idioma

So the ending -a does not always mean a noun is feminine.

Because standard modern Spanish uses the short possessive directly before the noun, without an article:

• mi madre = my mother
• tu hermano = your brother
• su casa = his/her/their house

Using la mi madre is not normal in standard modern Spanish.

Also, Spanish usually does not use the article with these short possessives when they come before the noun.

Yes, but it changes the nuance slightly.

• Mi madre es realista = describes her as realistic in general
• Mi madre es una realista = emphasizes that she is a realist, almost as a type of person

In your sentence, es realista sounds more natural and straightforward because it focuses on her attitude, which is then explained by the second part of the sentence.

Yes, but idioma is the more straightforward choice here.

• aprender un idioma = to learn a language
• aprender una lengua = also possible

In many contexts, idioma refers more clearly to a language such as Spanish, English, or French. Lengua can also mean tongue, and depending on context it can sound a bit more technical or linguistic.

So for a general sentence like this, idioma is a very natural choice.