You don't have to flash here is some guide:
1) download you language (here or from iprotebe.cz)
2) go to tpa/preset/system/language
using FAR and copy your language here
3) delete lng.dat and lng.lst
4) change or add your language in tpa/preset/custom/customize.xml:
to the section about languages:
<allowed-language>hr</allowed-language>
<?xml version="1.0" encoding="UTF-8"?>
<customization>
<info>
<cda-number>CDA102430/2</cda-number>
<cda-revision>R2A</cda-revision>
<model>W800i</model>
<operator>Generic</operator>
<language-variant-name>EMEA_2</language-variant-name>
<allowed-language>en</allowed-language>
<allowed-language>cs</allowed-language>
<allowed-language>hr</allowed-language>
<allowed-language>sr</allowed-language>
<allowed-language>sl</allowed-language>
<allowed-language>sk</allowed-language>
Or simply delete any language from tpa/preset/system/language directory and copy there your language with the name of that deleted.