When you drink potion of speed you get haste. Haste has these effects, "Target's speed is doubled. It also gains a +2 bonus to AC, advantage to Dex. saves and 1 additional action." so you get one extra main hand attack.
You can buy invisibility potions from Auntie Ethel from Druid Grove and Roah Moonglow from Shattered Sanctum (Goblin camp). When you take long rest, merchants usually replenish their stock so you can buy more potions after resting.
Can you change the language setting in BG3 or how do you manage to play it if you don't understand english?
