I have been using Codestyling Localization (CL) plugin successfully for long, translating texts into cs_CZ.
When translating, CL uses correct plural form for my language, yet after saving translations with 'Create MO' the plural form in .po file is not correct. It changes into the English plural form i.e.
"Plural-Forms: nplurals=2; plural=n != 1;\n"
As a result of this the plurals displayed at frontend are not correct!