Ths is not B/C and will change the display when the switcher is placed in a column.
Example:
before:

after

I'd class this as a B/C break as you are removing the reset for browser defaults on a UL. 3PD templates may need to add back this CSS if not already present within template styling.

If needed I would add padding/margin to the Protostar template CSS instead.

css is not part of the b/c promise

In that case delete media/mod_languages/css/template.css. There is no point of having any CSS that template developers can not rely on.

css is not part of the b/c promise

For a totally new release, maybe (4.0 for example), but certainly not for a current release line.

I am going by the documented policy and not simply inventing something

https://developer.joomla.org/development-strategy.html#backward_compatibility

It's not part of the B/C promise but if it's going to be disruptive, as is the case here, then we need a different solution.

Seems like that's going to be adding more CSS to account for when the language switcher is inside the .header-search div in the Protostar template.

Seems like that's going to be adding more CSS to account for when the language switcher is inside the .header-search div in the Protostar template.

good idea. I guess these should be in protostar template css.

We just need this in protostar imho:

.header-search .mod-languages ul {
	margin:0 0 5px 0;
}