This issue originated from gnu.org #319482.
This is pt_BR.dat:
special - -*- ' -*- . -** ª -** º --* µ *--
The word list contains the following word: "µg".
Initially pt_BR.dat was encoded in utf-8, not iso-8859-1 (my bad), and "make install" printed this error message:
"Warning: The word "µg" is invalid. The character 'µ' (U+B5) may not appear at the beginning of a word. Skipping word."
After converting pt_BR.dat to iso-8859-1, similar error messages vanished but this particular message changed to:
"Warning: The word "µg" is invalid. The character 'µ' (U+B5) may not appear in the middle of a word. Skipping word."
Note that now Aspell thinks "µ" is in the middle of the "µg" word, not in the beginning.
The preliminary dictionary tarball should be retrieveable from the previously named issue, or from Kevin Atkinson's hard disk drive.