This data (portuguese.dict, portuguese_synth.dict) is based on data
from FreeLing (http://nlp.lsi.upc.edu/freeling/), adapted to
LanguageTool by Tiago Santos (https://github.com/TiagoSantos81/FreeLing).

Quoting the FreeLing COPYING file:

"Portuguese dictionary contained in the files in
data/pt/dictionary (this package) and share/pt/dicc.src (after
installation) was extracted from the LABEL-LEX project
(http://label.ist.utl.pt/pt/labellex_pt.php) developed by
Laboratório de Engenharia da Linguagem (http://label.ist.utl.pt)
at Universidade Técnica de Lisboa (http://www.utl.pt), and
adapted to FreeLing by Pablo Gamallo and Marcos Garcia, from
Universidade de Santiago de Compostela.  It is distributed under
its original GPLv3 license."

Thus, this data is licensed under GPL version 3. Please note that LanguageTool
does not use any code from FreeLing, only data. This means the general FreeLing
license (which has changed from LGPL to GPL, then to AGPL over the years)
does not apply to this.
