Polish language spell check stop working in TexStudio 2.11, after migration from 2.10.8. It is removed from default language list in language checking configuration tab and it is also imposible to install it again (the language does not appear after installation). The downgrade to 2.10.8 make it works again.
fixed: hg 6085 (1ee815297343)
As a workaround, you can directly place the .dic and .aff file in [settings-dir]/dictionaries. There was a recursive scan for the files in subdirectories, which was removed in 2.11.0 due to perfromance reasons. The fix will recurse again but only one subdirectory down.