Re suggestions for the best way to teach someone with minor Aphasia, would be as you would with any student. Ideally to design the teaching to take advantage of how they find learning best for them, and where their interests lie. i.e. also to consider issues such as Are You an Auditory or Visual Learner || Tips, and other such variations.
I can tell you about my experience with my son, who had learning difficulties. He had challenges with speaking. And challenges with reading.
When he was about 14 years, it was suggested by learning experts that we saw that it could beneficial for him to learn a second language. That a second language would be "mapped" to a different part of his brain. And that he may become more proficient with his second language than he may be able to become with his first.
Language learning, however, is a complex procedure that scientists have determined is not limited to any hemisphere of the brain, but instead involves information exchange between the left and the right sides.
The part of the brain where humans store a second language varies according to the age they acquire it. ... in adult learners it is saved in a different area of the brain. ... When it comes to the corpus callosum, for instance, several studies suggest that the data transfer between the left and the right hemispheres that happens during the acquisition of a second language contributes to an increase in its white matter volume and in the number of fibers that provide greater cortical connectivity.
T. J. Kennedy ; 2006 LanguageLA, Language Learning and Its Impact on the Brain: Connecting Language Learning with the Mind Through Content‐Based Instruction},
The brain is a complex organ. There are instances where even when people have lost half their brain, that the remaining half can take over functions. In the case of a young girl named Cameron Mott, who suffered a rare and debilitating disease of Rasmussen's Encaphalitis, which would lead to paralysis and eventually death, her neurologist suggested drastic surgery of removing an entire half of Cameron's brain.
The long term effects of this amazingly slight, and she has no problems understanding language, music, math, stories. She participates in sport and is successful in school.
; The brain: The story of you D Eagleman Canongate Books, 2015 - Chapter 6 Who will we be?
This is not an area that I am expert in. However I thought this knowledge that I have studied may be applicable to address the concerns you have expressed.
Though I would also suggest you statement :
They are right-handed, so essentially left-brain language learning is not possible.
That this may be a fact that, in my reading, some may also question. From what I read, there is still so little we understand with how our brain works. And that different people have different mind maps. That information can be mapped onto different parts of the brain of one individual to another.
The Brain is like a City
Just like a city, the brain’s overall operation emerges from the networked
interaction of its innumerable parts. There is often a temptation to assign a
function to each region of the brain, in the form of “this part does that”. But
despite a long history of attempts, brain function cannot be understood as the
sum of activity in a collection of well-defined modules.
Instead, think of the brain as a city. If you were to look out over a city and
ask “where is the economy located?” you’d see there’s no good answer to the
question. Instead, the economy emerges from the interaction of all the elements
– from the stores and the banks to the merchants and the customers.
And so it is with the brain’s operation: it doesn’t happen in one spot. Just as
in a city, no neighborhood of the brain operates in isolation. In brains and in
cities, everything emerges from the interaction between residents, at all scales,
locally and distantly.
Ref: The brain: The story of you D Eagleman Canongate Books, 2015 - Chapter 2 What is Reality?
Regarding your question :
Would certain language be easier to grasp than others (Mandarin vs Spanish for instance?) Their first language is English.
I would suggest it would be easier to learn a language which had similar sounds to English. As even sounds we hear, we need to learn to hear them. As well as to learn how to produce them.
a baby born in Japan and a baby born in America can hear and respond to all the sounds in both languages. Through time, the baby raised in Japan will lose the ability to distinguish between, say, the sounds of R and L, two sounds that aren’t separated in Japanese. We are sculpted by the world we happen to drop into. ref: The brain: The story of you D Eagleman Canongate Books, 2015 - Chapter 1 Who am I?
Though I would also suggest, the best second language to learn is one that they could be interested in learning and motivated to learn.
You and/or they may also be interested in checking Language Difficulty Rankings for English speakers.