Serbian Latin localization not found on rendering (xsl stylesheets1.66.1)

Hi group.
When trying to render DocBook documents (say, article) that has lang 
attribute set to "sr_Latn", I get output as if that attribute is set to 
"sr", that is, style sheets do not find that localization and use a 
fallback (sr). After doing some tracing, i think that problem might be 
in the following segment of common/i10n.xsl (lines 68-70 in 
docbook-xsl-1.66.1 package)

----- start of inserted text ------
<xsl:variable name="language" select="translate($mc-language,
                                         'ABCDEFGHIJKLMNOPQRSTUVWXYZ',
                                         'abcdefghijklmnopqrstuvwxyz')"/>
------ end of inserted text ------

which (if I understand correctly), lowercases language name to find its 
common elements localization in $LANG.xml file. In sr_Latn.xml, language 
is identified as language="sr_Latn"; however, if I change it to 
language="sr_latn", it gets found, and common elements output is correct.
I also tried some older xsl style sheets packages (which have sr as 
"Serbian in Cyrillic Script" and sr_Latn as "Serbian in Latin Script") 
and found the same problem in all of them.
Is this fix (lowercasing language name in LANG.xml) valid (not breaking 
IANA standard etc)?

TIA, Branislav Jovanovic.

-- 
== Branislav (Brane) Jovanovic' ===  counter.li.org #132905 ==
Ducharme's Precept:
	Opportunity always knocks at the least opportune moment.