En español, casi todas las sílabas tienen al menos una vocal.

When Spanish names a language after en, it usually does not use the article.

• en español = in Spanish
• en inglés = in English
• en francés = in French

You can use el español when you mean the Spanish language as a noun:

• El español es una lengua románica. = Spanish is a Romance language.

But in this sentence, en español is the normal choice.

In Spanish, names of languages are normally written with a lowercase letter.

So:

• español
• inglés
• francés

This is different from English, where language names are capitalized. Spanish only capitalizes them at the beginning of a sentence or in certain title styles.

Casi todas means almost all.

• casi = almost
• todas = all, feminine plural

So:

• casi todas las sílabas = almost all syllables

A useful point: casi never changes form, but todo / toda / todos / todas changes to match the noun.

Because sílaba is a feminine noun.

Its forms are:

• la sílaba = the syllable
• las sílabas = the syllables

Anything that goes with it must match in gender and number:

• toda la sílaba
• todas las sílabas

So todas is feminine plural because sílabas is feminine plural.

Spanish uses the definite article more often than English, especially when talking about things in general.

So Spanish often says:

• Las sílabas...
• Los perros...
• Las vocales...

where English may simply say:

• Syllables...
• Dogs...
• Vowels...

In this sentence, casi todas las sílabas is the natural Spanish way to say almost all syllables.

Because the subject is plural:

• casi todas las sílabas = plural

So the verb must also be plural:

• tienen = they have

If the subject were singular, you would use tiene:

• La sílaba tiene...

This is standard subject-verb agreement.

Al menos means at least.

It is a fixed expression, so it is best learned as a whole unit.

Examples:

• al menos una vocal = at least one vowel
• al menos dos minutos = at least two minutes
• al menos una vez = at least once

A very similar expression is por lo menos, which also means at least.

Because after at least one, Spanish uses the singular noun, just like English does.

• al menos una vocal = at least one vowel

If the sentence meant more than one, it would be plural:

• al menos dos vocales = at least two vowels

So the singular here is completely normal.

Yes. In grammar, vocal means vowel.

That can be confusing for English speakers, because English vocal usually means something related to the voice or singing. In Spanish, vocal can have other meanings in other contexts, but in grammar it definitely means vowel.

So here:

• una vocal = a vowel

A rough guide is:

• español → es-pa-NYOL
• sílabas → SI-la-bas
• vocal → bo-KAL

A few useful details:

• ñ sounds roughly like ny in canyon
• the stress is on -ñol in español
• the stress is on sí- in sílabas
• Spanish b and v are pronounced very similarly, so vocal starts with a sound close to b

These are only approximations, but they are helpful for an English speaker.

That mark is called a tilde, and ñ is considered a separate letter in Spanish, not just n with decoration.

So:

• n and ñ are different letters
• ano and año are different words

In español, the ñ is an essential part of the spelling and pronunciation.

Because of Spanish stress rules.

Sílabas ends in -s. Words ending in a vowel, -n, or -s are normally stressed on the second-to-last syllable. But sílabas is stressed on the first syllable, so it needs a written accent:

• SÍ-la-bas

By contrast:

• casi ends in a vowel and is stressed on the second-to-last syllable, so it follows the normal rule and needs no accent
• vocal ends in -l, so it is normally stressed on the last syllable, which is exactly what happens, so it needs no accent

Yes. You could also say:

Casi todas las sílabas tienen al menos una vocal en español.

That is also natural and correct.

The difference is mostly about emphasis and flow:

• En español, ... puts the language context first
• ... en español gives the main statement first and adds the context at the end

The comma after En español marks it as an introductory phrase. With short introductory phrases like this, the comma is often a style choice, but it is perfectly normal here.