Add Polish language

[Jendker]

Added Polish language.

There are some differences in project.pbxproj, which I don't understand, like (lines 71 and 72 ea9ed05#diff-0556d61b816fdb7b583fc02e76b28931L71):
FA6DD5141C7DEDB600317E73 /* (null) in Resources / = {isa = PBXBuildFile; };
FA6DD5151C7DEDB600317E73 / (null) in Resources */ = {isa = PBXBuildFile; };

I would need confirmation that this is fine.

Apart from that I could not verify the changes in the running program as I am sitting already on Mac OS Mojave with Xcode 10.1 where Clipy does not build. But I made the translation based on English and German, so it should be of good quality.

The error during build is:

PhaseScriptExecution [CP]\ Embed\ Pods\ Frameworks /Users/Jedrzej/Library/Developer/Xcode/DerivedData/Clipy-gavyfmprnrhelzcgtqylwajzjyir/Build/Intermediates.noindex/Clipy.build/Debug/Clipy.build/Script-07B47785AE0AC2185B570C28.sh (in target: Clipy)
    cd /Users/Jedrzej/GitHub/Clipy
    /bin/sh -c /Users/Jedrzej/Library/Developer/Xcode/DerivedData/Clipy-gavyfmprnrhelzcgtqylwajzjyir/Build/Intermediates.noindex/Clipy.build/Debug/Clipy.build/Script-07B47785AE0AC2185B570C28.sh

mkdir -p /Users/Jedrzej/Library/Developer/Xcode/DerivedData/Clipy-gavyfmprnrhelzcgtqylwajzjyir/Build/Products/Debug/Clipy.app/Contents/Frameworks
rsync --delete -av --filter P .*.?????? --filter "- CVS/" --filter "- .svn/" --filter "- .git/" --filter "- .hg/" --filter "- Headers" --filter "- PrivateHeaders" --filter "- Modules" "/Users/Jedrzej/Library/Developer/Xcode/DerivedData/Clipy-gavyfmprnrhelzcgtqylwajzjyir/Build/Products/Debug/AEXML/AEXML.framework" "/Users/Jedrzej/Library/Developer/Xcode/DerivedData/Clipy-gavyfmprnrhelzcgtqylwajzjyir/Build/Products/Debug/Clipy.app/Contents/Frameworks"
building file list ... done
AEXML.framework/
AEXML.framework/AEXML -> Versions/Current/AEXML
AEXML.framework/Resources -> Versions/Current/Resources
AEXML.framework/Versions/
AEXML.framework/Versions/Current -> A
AEXML.framework/Versions/A/
AEXML.framework/Versions/A/AEXML
AEXML.framework/Versions/A/Resources/
AEXML.framework/Versions/A/Resources/Info.plist

sent 212100 bytes  received 106 bytes  424412.00 bytes/sec
total size is 211658  speedup is 1.00
/Users/Jedrzej/GitHub/Clipy/Pods/Target Support Files/Pods-Clipy/Pods-Clipy-frameworks.sh: line 104: EXPANDED_CODE_SIGN_IDENTITY: unbound variable
Command PhaseScriptExecution failed with a nonzero exit code