Could you please explain the meaning of "retaining wall relation"?

"there is a retaining-wall relation between language and group. "

I looked up the retaining wall (Retaining walls are relatively rigid walls used for supporting soil laterally so that it can be retained at different levels on the two sides. Retaining walls are structures designed to restrain soil to a slope that it would not naturally keep to (typically a steep, near-vertical, or vertical slope.)

However, I found it hard for me to understand what kind of relation is retaining wall relations?

This question is about an analogy.

Soil on a slope has a natural tendency to disperse downhill and is prevented from doing so by the boundary of a retaining wall.

Languages have a natural tendency to disperse by intermarriage, cultural exchange, conquest, trade and migration. They may be prevented from doing so by geographical boundaries such as rivers, deserts, mountain ranges and oceans).

More relevant to this question are the boundaries to the dispersion of languages imposed by human groups. Nation states, ethnic groups, tribes, religious groups and others may create boundaries to the dispersion of languages either by rigidly policed physical frontiers or by ingrained human (ethnic, religious, tribal) attitudes.

Whatever their nature, these human boundaries between groups inhibit the natural dispersal of language, in a way that is analogous to the inhibition of natural downslope soil movement by retaining walls.

I think this figurative idea has to be construed faithfully on the model of the literal meaning. In this meaning an element, earth, has a natural propensity to occupy as much space as is available, to spread to as much territory as there is in its proximity and thus to lose its initial spatial distribution, its shape. It is the wall that prevents that from happening. In the figurative meaning, particularising the analogy on the getting out of shape level, language (more or less loose earth), is found to be a mass with the tendency to lose its function, to be eroded by various agents and the group action to prevent that is that of a propping up by a sheer forcing into "shape". Here the action of the group and the group are identified so that the group is the wall.

However, from what a comment suggests to me (user Jim) the same relation in the other way is quite an appealing possibility. Possibly, both relations have been meant.