We're using as many defaults from the configure step as we can. We're also opinionated upon the defaults, but obviously allow most compare-locales options to be specified.
There are two exceptions:
Reference language is specified to be en-US, without optional argument. This is our in-tree command, and the reference language is known.
We always clobber the merge dir, and don't give an option not to. We default to a merge dir in the objdir, so we don't need to be that paranoid as in the standalone version.
Also, compare-locales clobbers merge-dir/browser etc, so you're not going to get / removed.