"I speak English."
Tá Béarla agam states an ability to speak English (or whichever language is being referred to). So someone living in Germany who studied English in high school, but who hasn't actually spoken a word of it in years can say "I speak English"/Tá Béarla agam, but they can't (truthfully) say Labhraím Béarla, because they don't actually speak English, even though they know how.
In other words, English is ambiguous. "I speak English" can be a claim of ability, or a statement about habitual actions. Equally, "I can speak English" can be a claim of ability, or a statement about having permission to speak English.