I already posted this bug under the info on LOP 3.5.5 release:
The smaller installer of LOP 3.5.5 does not contain Polish interface language pack. I have just installed the update and it offers all previously available UI languages, but no Polish. Locale is automatically set to Polish, there are writing aids for Polish, but there is no language pack for user interface...
I looked into the paf file with 7-Zip and under \$_OUTDIR\libreoffice\program\resource\ and $_OUTDIR\libreoffice\share\registry\directory I found all language files but Polish. Seems sth went wrong either in LO.org or in PA.com when packaging the suite...