Learn English – Is any difference between “speak English” and “speak in English”

The short answer is: of is a very flexible word.

I will speak on love.
I will speak about love.
I will speak of love.

These three expressions mean essentially the same thing.

You already know about speak about, but:

• To speak on X can mean "speak on the topic of X" – although, when it comes to using on, it might be more common to hear, "I will give a talk on X."
• To speak of X can also mean "speak about X," although it has a slightly more smooth and literary feel to it.

If anything, speak of seems to be more prevalent than speak about, at least in written texts – if you want to believe the ngram:

'English' is a word used both for the language and the nationality (of people who hail from England). So, when we say "I am an English teacher.", People may take the speaker as a teacher who lives in England. When we phrase it like - "I am a teacher or English language.", it becomes pretty much clear. Well, these days, both the notions are in play so you can use any of them.