Me siento cada vez más tranquilo cuando hablo en español.

Because the verb here is sentirse, which means to feel in the sense of your emotional or internal state.

• me siento tranquilo = I feel calm
• siento by itself usually means I feel/sense something, as in siento frío = I feel cold, or lo siento = I’m sorry

With an adjective like tranquilo, Spanish normally uses sentirse:

• Me siento tranquilo = I feel calm

So me is not optional here; it is part of the verb sentirse.

They are similar, but not exactly the same.

• Estoy tranquilo = I am calm
• Me siento tranquilo = I feel calm

Estar describes a state.
Sentirse emphasizes how that state feels to you personally.

In this sentence, me siento sounds very natural because the speaker is talking about their growing comfort level from the inside.

Cada vez más means more and more or increasingly.

So:

• cada vez más tranquilo = more and more calm
• literally, something like each time more calm

It is a very common Spanish structure:

• cada vez más fácil = easier and easier
• cada vez más difícil = more and more difficult
• cada vez más seguro = more and more confident

Because adjectives in Spanish agree with the person they describe.

• A male speaker would usually say tranquilo
• A female speaker would usually say tranquila

So the sentence changes depending on who is speaking:

• Me siento cada vez más tranquilo... = said by a male speaker
• Me siento cada vez más tranquila... = said by a female speaker

Because Spanish often leaves out subject pronouns when they are clear from the verb ending.

• siento already tells you it means I feel
• hablo already tells you it means I speak

So yo is unnecessary unless you want emphasis or contrast.

• Yo me siento cada vez más tranquilo... = more emphatic, like I feel more and more calm

The version without yo is the most normal one.

Because this sentence describes a habitual or general situation: whenever I speak Spanish, I feel more and more calm.

In Spanish, when cuando refers to something habitual, you use the indicative:

• cuando hablo en español = when I speak Spanish / whenever I speak Spanish

You would use cuando hable with the subjunctive if you were talking about a future event:

• Cuando hable en español mañana, estaré nervioso = When I speak Spanish tomorrow, I’ll be nervous

So here, hablo is correct because it is about a repeated experience, not a one-time future action.

Because Spanish uses the present tense for things that happen regularly or generally.

Here the idea is:

• When I speak Spanish, I feel more and more calm
• in other words, this is what usually happens

So hablo is not just about this exact moment. It can describe a habit, a repeated situation, or a general truth.

You could say that, but it is less natural in this context.

• cuando hablo en español = when I speak Spanish / when I’m speaking Spanish
• cuando estoy hablando en español = when I am in the middle of speaking Spanish

Spanish uses the simple present much more often than English does. In many cases, hablo already covers what English might express with either I speak or I’m speaking.

So cuando hablo en español is the most natural choice here.

Both are possible, but they mean slightly different things.

• Hablo español usually means I speak Spanish as an ability
• Hablo en español means I speak in Spanish, focusing on the language being used in that moment

In this sentence, the speaker is talking about the situation of using Spanish, so en español fits very well.

Cuando means when.

It introduces the time clause:

• cuando hablo en español = when I speak in Spanish

So the sentence has two parts:

• Me siento cada vez más tranquilo = I feel more and more calm
• cuando hablo en español = when I speak in Spanish

Together, the second part tells you when the first thing happens.

Only the adjective would normally change:

• Me siento cada vez más tranquila cuando hablo en español.

Everything else stays the same.
That is because tranquilo/tranquila must agree with the speaker.

A few useful pronunciation points:

• h in hablo is silent, so hablo sounds roughly like AB-lo
• español sounds roughly like es-pa-NYOL
• ñ is like the ny sound in canyon
• the written accent in español shows the stress falls on the last syllable: -ñol

So the whole sentence is approximately:

me SYEN-to KA-da BETH mas tran-KEE-lo KWAN-do A-blo en es-pa-NYOL

In most of Spain, z and soft c are pronounced with a th sound, but that does not affect any word in this sentence.