Windows: Fix NSIS language encoding (use UTF-8 instead of Win code pages)

Windows: Fix NSIS language encoding (use UTF-8 instead of Win code pages)

The old conversion to specific code pages caused display errors (see issue #2393 for Russian).

Based on bin/l10n.sh, the files in this commit were converted to UTF-8 using iconv with the following parameters:

  iconv -f CP1252 -t UTF-8 -o German.nsh German.nsh
  iconv -f CP1252 -t UTF-8 -o Galician.nsh Galician.nsh
  iconv -f CP1253 -t UTF-8 -o Greek.nsh Greek.nsh
  iconv -f CP1250 -t UTF-8 -o Slovenian.nsh Slovenian.nsh
  iconv -f CP1257 -t UTF-8 -o Estonian.nsh Estonian.nsh
  iconv -f CP1252 -t UTF-8 -o Italian.nsh Italian.nsh
  iconv -f CP1252 -t UTF-8 -o PortugueseBR.nsh PortugueseBR.nsh
  iconv -f CP1252 -t UTF-8 -o Spanish.nsh Spanish.nsh
  iconv -f CP1252 -t UTF-8 -o Dutch.nsh Dutch.nsh
  iconv -f CP1252 -t UTF-8 -o Finnish.nsh Finnish.nsh
  iconv -f CP932 -t UTF-8 -o Japanese.nsh Japanese.nsh
  iconv -f CP1250 -t UTF-8 -o Slovak.nsh Slovak.nsh
  iconv -f CP1254 -t UTF-8 -o Turkish.nsh Turkish.nsh
  iconv -f CP1252 -t UTF-8 -o Norwegian.nsh Norwegian.nsh
  iconv -f CP852  -t UTF-8 -o Czech.nsh Czech.nsh

The installer script nextcloud.nsi in the client-building repo needs the following line (to be added on top):
  Unicode true

Signed-off-by: Michael Schuster <michael@schuster.ms>