I´d say we both agree that broadening our horizons, which inludes less bigotry, less prejudices, etc., is a good thing in itself. If language learning, traveling, or studying / living abroad contribute to that - wonderful!
However, every communication can reject another communication and hence become a conflict. For example, we´re having a conflict right now about the moderating effect of “language learning” on conflicts 
Or another example:
I love my family. But there have been many conflicts in the past (about my behavior at school when I was young, what I should study at university, what I should do with my life, etc.), even though we are very close! New conflicts pop up from time to time, and there are a few (unsolvable) conflicts that have been going on for decades.
Does this have anything to do with language learning and a better understanding of each other? Not at all. We´re all native speakers, almost all of the children have an academic background, we share many of the same values and traditions - and we love each other.
Therefore:
Conflicts are a completely normal part of “all” communication processes - beyond politics! In other words, there are conflicts everywhere and all the time in families, in and between organizations, in everyday interactions, etc.
Complexity research even attributes a very positive role to conflicts because they enable social systems to learn and better adapt to a changing environment!
Unfortunately, conflicts are a “dangerous” social learning mechanism because they can easily turn violent (violence being understood here as a “solution” to end conflicts that have intensified and are permanent).
We´ve known this for tens of thousands of years. And that´s why societies all over the world have developed more peaceful conflict management mechanisms like hierarchy (that is able to absorb conflicts, esp. in organizational structures), a highly complex legal system whose basic societal function is “conflict management” (!), the state´s monopoly of force in modernity, diplomatic negotiations, etc.
When many or all of these social/societal conflict management mechanisms fail, and therefore conflicts become increasingly intense, violence (including war) becomes more likely.
"The most critical thing I’m willing to say about the hypothesis is that it’s untestable. "
I don’t see how “language learning” can be part of the mix of reliable conflict resolution mechanisms that have evolved over the last tens of thousands of years, because compared to the effectiveness of the principle of hierarchy, the legal system, the state monopoly on the use of force, etc., its potential for preventing / managing a conflict is almost negligeable.
“the extent to which voters control the course of public policy is a complex, not simple, question”
Yes, and the topic of “uneven policy responsiveness” is an ongoing research effort in the social sciences.
On the other hand, we know from complexity research that (adaptive) complex systems tend to produce Pareto distributions (e.g. 80-20, 90-10, etc.) in the long run. And this seems to be the case also in the economic and political systems of our time.
This isn´t restricted to democracies, but includes all kinds of regimes (monarchies, democracies, and authoritarian regimes).
"One can easily make a strong argument that public action/ opinion has played a big role in shaping many conflicts past and present. "
Well, the career of “public opinion” started basically in the (early) modern era (let´s say in the 17th / 18 th centuries). And then mainly in democratic societies. In contrast, authoritarian and dictatorial regimes usually suppress and control public opinion.
To make things easier, we could focus exclusively on the 20 / 21th century. Let´s say the War in Vietnam between the US and the Vietcong / North Vietnamese.
A simplicistic narrative here might be: Public opinion in the US was against the Vietnam War - and that´s why the war ended in 1975.
However, there are many more factors at play in this complex constellation:
- It could never break the supply lines
- It couldn´t attack China (China in the Vietnam War - Wikipedia)
- It couldn´t use limited nuclear weapons
- They US Army´s decline itself: “Racial incidents, drug abuse, combat disobedience, and crime reflected growing idleness, resentment, and frustration… the fatal handicaps of faulty campaign strategy, incomplete wartime preparation, and the tardy, superficial attempts at Vietnamization.” (Shelby Stanton, Vietnam War - Wikipedia)
etc.
- The misconception that the U.S. Army could help build nations (as it tried in the last war in Afghanistan, too!)
- The use of false statistics (body counts!) and war game simulations based on unreliable data
- The ambivalence of US tactics itself. For example: " This policy of attempting to win the hearts and minds of the Vietnamese people, however, often was at odds with other aspects of the war which sometimes served to antagonize many Vietnamese civilian " (Opposition to United States involvement in the Vietnam War - Wikipedia)
etc. etc.
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The role of the media, which were able to report more or less “uncensored”:
Vietnam War and the media | History, Walter Cronkite, Photographers, & Facts | Britannica
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The corrupt South-Vietnamese dictatorship under President Ngô Đình Diệm:
" Many feel that [Diem] is unable to rally the people in the fight against the Communists because of his reliance on virtual one-man rule, his tolerance of corruption extending even to his immediate entourage, and his refusal to relax a rigid system of public controls." (from the “Pentagon Papers”, https://en.wikipedia.org/wiki/South_Vietnam#History)
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The determination and strategies of North Vietnam
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Even the growing opposition in the US itself had the support of many organized groups, i.e. the African-American civil rights, feminist, and Chicano movements, sectors of organized labor, organized vets of the Vietnam war, etc. (Opposition to United States involvement in the Vietnam War - Wikipedia).
etc.
In brief: Is public opposition to the war in Vietnam a factor here?
Definitely (and the Taliban learned this lesson well during the protracted war in Afghanistan!)
However, a disgruntled (disorganized) public opinion alone would never have been enough to end this war in an instant (see your quote from Lloyd George)!