RESOLUTION
Language for AWStats is defined by "Lang" directive in it's configuration files.New language have to be set in the main AWStats configuration files:
/etc/awstats/awstats.conf
/etc/awstats/awstats.model.conf
And the files that were already configured by Plesk for every domain which can be found in /usr/local/psa/etc/awstats/.
By default Lang is set to Lang="auto", it means the first available language accepted by browser.
You can change it to any supported value that can be found above the Lang directive in the configuration files:
# Possible value:
# Albanian=al, Bosnian=ba, Bulgarian=bg, Catalan=ca,
# Chinese (Taiwan)=tw, Chinese (Simpliefied)=cn, Croatian=hr, Czech=cz,
# Danish=dk, Dutch=nl, English=en, Estonian=et, Euskara=eu, Finnish=fi,
# French=fr, Galician=gl, German=de, Greek=gr, Hebrew=he, Hungarian=hu,
# Icelandic=is, Indonesian=id, Italian=it, Japanese=jp, Korean=ko,
# Latvian=lv, Norwegian (Nynorsk)=nn, Norwegian (Bokmal)=nb, Polish=pl,
# Portuguese=pt, Portuguese (Brazilian)=br, Romanian=ro, Russian=ru,
# Serbian=sr, Slovak=sk, Slovenian=si, Spanish=es, Swedish=se
# Turkish=tr, Ukrainian=ua, Welsh=cy.
for example for English:
Lang="en"
English
Chinese
French
German
Japanese
Russian
Spanish