How to reproduce:
Install "LibO_3.4.2_MacOS_x86_install_en-US.dmg" and "LibO_3.4.2_MacOS_x86_langpack_cs.dmg". Set Czech (Čeština) into preffered locale settings (not depend on UI setting, you can leave it on English US). Open any Czech document with grammar mistakes and go to menu Tools->Spelling and Grammar (F7).
Process without any corrections - Spelling and Grammar dialog appear, in front appear dialog "Continue checking from beginining of document?" you can clikc to Yes and Spellchecker finished without any corrections.
Normal process Grammar and spelling function and find my mistakes.
This bug was tested only on Mac OS X 10.7.0, but it should occure on all supported platforms. In czech discussion I've found WindowsXP users which found the same bug (http://www.zive.cz/poradna/libreoffice-33-a-34--nefukcni-kontrola-pravopisu/sc-20-cq-455908/default.aspx?consultanswers=1).
With limitations yes, you can download extension to OpenOffice (http://www.liberix.cz/doplnky/slovniky/ooo/dict-cs-2.0.oxt) but in find a lot of widely-known phases and spellchecker offer you replace correct form "při" with form with bad encoding "pØi". You might use a cocoAspell or any similar system-based spellchecker, but I didn't try it.
Have you updated your LibO 3.4.2 from a previous version? If yes, see Bug 37195 and let us know if is your case.
(In reply to comment #1)
> Have you updated your LibO 3.4.2 from a previous version? If yes, see Bug 37195
> and let us know if is your case.
No, this is a different case. Czech spell checker dictionary is not included in LibreOffice source, so we don't ship it at all.
So, exist any workaround for Libre Office now?
Or I must wait to fix it?
fixed in master (3.5)
Verified in LO 3.5.0 Beta 2