You can always go back and play Fallout 1 and 2. Oh and there is a game called Encased, which was sponsored by Larian actually - that does allow you to have low intelligence speech.
I think these days that sort of thing is considered to be in bad taste. It's essentially punching down.
Also your lowest possible int score is an 8 which doesn't really lend itself to inability to speak coherently. You can hit 7 with Volo's eye but that will just make your memory a bit worse.
1 (–5): Animalistic, no longer capable of logic or reason. Behavior is reduced to simple reactions to immediate stimuli
2-3 (–4): Rather animalistic. Acts on instinct but can still resort to simple planning and tactics
4-5 (–3): Very limited speech and knowledge. Often resorts to charades to express thoughts
6-7 (–2): Has trouble following trains of thought, forgets most unimportant things
8-9 (–1): Misuses and mispronounces words. May be forgetful
10-11 (0): Knows what they need to know to get by
12-13 (1): Knows a bit more than is necessary, fairly logical
14-15 (2): Fairly intelligent, able to understand new tasks quickly. Able to do math or solve logic puzzles mentally with reasonable accuracy
16-17 (3): Very intelligent, may invent new processes or uses for knowledge
18-19 (4): Highly knowledgeable, probably the smartest person many people know
20 (5): Famous as a sage and genius. Able to make Holmesian leaps of logic