The custom install rules in locale/client/Makefile.am are incorrect.
- It's trying to copy the .po files. Those are source code; it should install the .mo files.
- It doesn't work with out-of-source builds. The makefile copies the files from the current directory, which isn't necessarily the source directory, where the files actually are. The only file in build_dir/locale/client is the Makefile.
To reproduce 2., do
mkdir build;
cd build;
../configure --prefix=~/local/boinc-test; #or wherever...
cd locale/client;
make install; # this looks in build/locale/client/<lang> instead of locale/client/<lang>