Cuanto más practico español, más crece mi confianza.

Because here cuanto is a linking word in the correlative structure cuanto más…, más… (= the more…, the more…). It’s not a direct or indirect question, so it has no accent.
You use cuánto (with accent) in questions/exclamations: ¿Cuánto practicas? / ¡Cuánto practicas!

It uses the “double comparative” (correlative comparative) pattern:
Cuanto más + verb…, más + verb…
Meaning: As X increases, Y increases.

The first más intensifies the first clause (cuanto más practico… = the more I practise…).
The second más introduces the result/comparison in the second clause (más crece… = the more (it) grows…).
They work as a pair in this structure.

It’s standard and strongly recommended: Cuanto más practico español, más crece mi confianza.
The comma cleanly separates the two linked clauses. You may see it omitted in informal writing, but it’s better to keep it.

In this correlative structure, Spanish typically places más at the start of the second clause: …, más + verb…
You can also say …, mi confianza crece más, but it sounds less “set-phrase-like” and slightly less natural than the classic …, más crece mi confianza.

practico (no accent) = I practise (present tense of practicar).
práctico (with accent) is usually an adjective meaning practical: un método práctico.
So the accent changes both stress and meaning.

Both are possible, but practico español is very common and natural when talking about practising a language in general.
Adding the article (el) can sound a bit more “the language as a subject/thing”: Practico el español cada día, but it’s not required.

In Spanish, names of languages are not capitalized: español, inglés, francés.
They’re only capitalized if they start a sentence or are part of an official title.

Crecer (to grow) is very natural with abstract nouns like confianza.
You can also say aumenta mi confianza (my confidence increases)—that’s also correct, just a slightly different nuance (more “increase” than “grow”).

Not in a general factual statement like this. Present indicative is normal: Cuanto más practico…, más crece…
You might use other tenses depending on meaning, e.g. future: Cuanto más practique, más crecerá mi confianza (more like the more I practise, the more my confidence will grow).

In most of Spain (with distinción), z is pronounced like English th in think, so confianza sounds like kon-FYAN-tha.
español has ñ, pronounced like ny in canyon: es-pa-NYOL.