Doubting is a state-of-mind just as a belief is a state-of-mind, and one can show states-of-mind apart from language.

I would agree that one can show states of mind without using language, if that's what you mean by "apart from language". However, can one be in a state of mind that doubts his/her own belief without language?

What sort of doubt can one have if one has no language?

Disbelief and doubt.

What's the difference in an agent that has no language?

Both belief and doubt are conceptual, actions are inadequate to express these.
An observer might infer them from the actions of one whose language is not developed, such as a child having an idea that a treat under a cup, but the belief that the treat is under the cup is not a belief in the child.

↪creativesoul Both belief and doubt are conceptual, actions are inadequate to express these.

An observer might infer them from the actions of one whose language is not developed, such as a child having an idea that a treat under a cup, but the belief that the treat is under the cup is not a belief in the child. — charleton

I disagree that belief is conceptual. At least, not all belief is conceptual. That is... assuming that being conceptual requires consisting in/of language.

If the language-less child believes that there is a treat under the cup, then it's belief cannot consist of the terms used to report upon it. What else could it consist in/of.

If the language-less child believes that there is a treat under the cup, then it's belief cannot consist of the terms used to report upon it. What else could it consist in/of — creativesoul

Maybe there is conceptual belief and doubt of the Cartesian kind. And maybe there is an existential belief and or doubt that is more basic than its conceptual derivative.

Belief would need to be accrued then. Starting simply and gaining in complexity. The simple could not have propositional/linguistic content. What else could belief consist in/of?

To the topic...

Being a better person is a neverending project. Thus, failing to meet one's own moral standards isn't necessarily an objectionable failing. It's part of the process.

Both belief and doubt are conceptual, actions are inadequate to express these. An observer might infer them from the actions of one whose language is not developed, such as a child having an idea that a treat under a cup, but the belief that the treat is under the cup is not a belief in the child. — charleton

Belief and doubt are only conceptual if you believe that they're products of language. However, it's been the case generally that beliefs (I would also add doubts) are states-of-mind, and states-of-mind are not dependent on linguistic concepts. It's not as though primitive man, before the advent of language, didn't have beliefs and/or doubts. Part of what it means to have a language is to put thoughts into words, thinking is prior to language, and prior to concepts.

An aomeba can demonstrate what you like to call belief. When it envelops a piece of food you say "it knew the food was there". This is absurd anthropomorphisation.

When a fledgling flaps its wings for the first time the fool says "all it has to do is believe it can fly and it shall".
A shark has no need of belief when following a trail of blood finds food; it just does it.

What's absurd is that you think that you can apply this idea to an amoeba. Amoeba's don't have the complexity of actions that humans have. Moreover, not all actions that lifeforms perform show beliefs. Not all of our (modern man's) actions show beliefs. If we were to watch primitive humans build things we could infer that they believe certain things by observing their actions. For example, they might go to a certain area of the forest and cut down trees and reshape them to build something, or they might measure one log by laying it alongside another, confirming that they are roughly the same size.

How does one even get to a language without having beliefs prior to having a language?

Obviously there is a difference between actions that are instinctual and one's that are not. How does one know the difference - by observation.