What did the Buddha say about motivation?

He said "whatever motivates one on the path is good."

Is there a difference in the Pali language between "intention" (as in Right Intention) and "motivation"?

I think that "right intention" (or "resolve") is connected with a sense of purpose -- with "purpose" meaning, for example, "I do this, in order to (for the purpose of, with the intention to) accomplish that".

I think that "right intention" is the second factor of the path and that it's closely related with "right view". I think that "right view" would explain why you want to do something, and could therefore be described as the "motive" for subsequent intention.

Similarly "wrong view" conditions wrong intention and so on.

harmlessness seems to be more passive and therefore maybe not a motivation in the same way

One of the doctrines of harmlessness is Chapter 10 of the Dhammapada, the first verse:

All tremble at violence; all fear death. Putting oneself in the place of another, one should not kill nor cause another to kill.

To relate that to intention, I'd say that "Putting oneself in the place of another" is a view, and that the consequence of that view is the "intention" to be harmless (and, consequently, harmless actions).

Also I think that "harmlessness" and "good-will" are closely related, two sides of the same coin. I can barely tell them apart. Also you say it's "not a motivation" but I think that's a motive of all ethics (sila):

[By abandoning the taking of life, abandoning stealing, abandoning lying, illicit sex, and the use of intoxicants] ... the disciple of the noble ones ... gives freedom from danger, freedom from animosity, freedom from oppression to limitless numbers of beings. In giving freedom from danger, freedom from animosity, freedom from oppression to limitless numbers of beings, he gains a share in limitless freedom from danger, freedom from animosity, and freedom from oppression ... [these are the five gifts] — original, long-standing, traditional, ancient, unadulterated, unadulterated from the beginning — that is not open to suspicion, will never be open to suspicion, and is unfaulted by knowledgeable contemplatives & brahmans. And [these are the rewards of merit], reward of skillfulness, nourishment of happiness, celestial, resulting in happiness, leading to heaven, leading to what is desirable, pleasurable, & appealing; to welfare & to happiness.

You might be asking for the "sense of samvega" though (which is mentioned in Saptha Visuddhi's answer, see also for example here). I think that samvega is understood to mean not only a sense of dismay, perhaps as a result of view again, but also a sense of (purposeful) urgency.

And I think that samvega is associated with virya (translated as energy and, as you mentioned, effort).

Heedfulness motivates you. The Pali word is ‘APPAMADA’. Supreme Buddha Himself has said that all of what he has taught, if all that can be put into ONE word, it is ‘APPAMADA’. Putting into practice what you know of Dhamma right now, motivates tou. You will not postpone any acts or decisions for a ‘tomorrow’. Life is in the today – life is here and now. For many, this Dhamma Path has created new possibilities, that only in living in the present we can learn to live this truth.

Another factor that would motivate you in this Dhamma Path is Generosity. It helps you to be free from attachment. Giving motivates us to let go of our clinging. The more we practice giving the more we let go of our clinging. And one day, we can be free from all forms of clinging. Generosity also helps us collect merits. Merits, that come with us no matter where we go, like the shadow that never leaves.

One other factor that would motivate you in this Dhamma Path is a sense of samvega: a sense of dismay over the nature of the human condition because everybody is subject to these same problems, but no one has shown us how to deal with such. When you really see that there’s a connection between unskillful intentions and needless suffering, you become genuinely motivated to find the escape from that suffering.

Supreme Buddha’s advice for this is that of the practise of four activities: Listening to Dhamma, participation in Dhamma discussion, Samatha and Vipassana. Only the individual who practises these four activities develops his mind up to Arahantship. This is not a short distance run, this is a marathon like no other.

'Chanda' as an 'iddhipada', translated as 'zeal', 'desire', 'will', 'love'. I prefer 'devotion'.

Some suttas: SN 51.15 and SN 51.20.

Right effort(sammā-vāyāma) in terms of mindfulness is not a forceful , grit your teeth kind of effort. It's a moment by moment, continuous, good cyclical habit kind of effort.

Cetanā is Pali for

1- intention

2- volition

3- directionality of mind

4- attraction

5- urge

Intention(cetanā) can be defined as a mental factor that moves or urges the mind in a particular direction, toward a specific object or goal. Intention(cetanā) is the most significant mental factor involved in the creation of karma.

Sammā-vāyāma and cetanā are two different concepts. In practice, one concept(or the actual experience the concept points towards) might happen right after the other though. Then there is right intention (samma sankappa) that isn't really like sammā-vāyāma. Imho at least.

Samvega, the sense of shock and empty alienation that comes with realizing the futility and meaninglessness of living in the sleeping world, is what motivated the Buddha and others onto the wholesome eightfold path. Some are motivated by other things like faith(saddhā) in the Buddha's teaching.

Right effort and right view are the two most important factors for an insight meditator to understand, it seems to me.

Right effort can be something like, effort to continue striving to remember to focus awareness on whatever experience arises in awareness and to strive in a broader sense to transcend all assumptions even the ones we don't yet see.

The Four Supreme Efforts

(cattārimāni sammappadhānāni)

1-Restraint Not to let an unwholesome thought arise which has not yet arisen(anuppādāya)

2-Abandonment Not to let an unwholesome thought continue which has already arisen.(pahānāya)

3-Cultivation To make a wholesome thought arise which has not yet arisen.(uppādāya)

4-Preservation To make a wholesome thought continue which has already arisen.(ṭhitiyā)

Effort can come easier with interest but then the path is to eventually develop a certain disinterest.