This is reproducible 100% of the time by visiting https://commons.wikimedia.org/wiki/Commons:Mus%C3%A9e_des_Augustins/test2 .
Clearly somehow a variant of the uniq/qinu placeholder ends up getting treated as a language code. I'm not sure whether the root cause is a bug in Translate or core. However, Translate is not catching the MWException Message->inLanguage throws (it is documented to throw, though it's not documented *when* it throws).
Sample backtrace below. This seems similar to, but not the same as, bug 44608 (T46608).
2013-07-02 19:56:48 mw1044 commonswiki: [b172dcac] /wiki/Commons:Mus%C3%A9e_des_Augustins/test2 Exception from line 217 of /usr/local/apache/common-local/php-1.22wmf8/languages/Language.php: Invalid language code "[[:#if:]]uniq9aec59bbd6046bae-item-248--qinu"
#0 /usr/local/apache/common-local/php-1.22wmf8/languages/Language.php(196): Language::newFromCode('[[:#if:]]uniq9...')
#1 /usr/local/apache/common-local/php-1.22wmf8/includes/Message.php(381): Language::factory('[[:#if:]]uniq9...')
#2 /usr/local/apache/common-local/php-1.22wmf8/extensions/Translate/TranslateHooks.php(254): Message->inLanguage('[[:#if:]]uniq9...')
#3 [internal function]: TranslateHooks::translateMessageDocumentationLanguage(Array, '[[:#if:]]uniq9...')
#4 /usr/local/apache/common-local/php-1.22wmf8/includes/Hooks.php(196): call_user_func_array('TranslateHooks:...', Array)
#5 /usr/local/apache/common-local/php-1.22wmf8/includes/GlobalFunctions.php(3834): Hooks::run('LanguageGetTran...', Array)
#6 /usr/local/apache/common-local/php-1.22wmf8/languages/Language.php(873): wfRunHooks('LanguageGetTran...', Array)
#7 /usr/local/apache/common-local/php-1.22wmf8/languages/Language.php(918): Language::fetchLanguageNames('[[:#if:]]uniq9...', 'all')
#8 /usr/local/apache/common-local/php-1.22wmf8/includes/parser/CoreParserFunctions.php(770): Language::fetchLanguageName('fr', '[[:#if:]]uniq9...')
#9 [internal function]: CoreParserFunctions::language(Object(Parser), 'fr', '[[:#if:]]UNIQ9...')
#10 /usr/local/apache/common-local/php-1.22wmf8/includes/parser/Parser.php(3560): call_user_func_array(Array, Array)
#11 /usr/local/apache/common-local/php-1.22wmf8/includes/parser/Parser.php(3278): Parser->callParserFunction(Object(PPTemplateFrame_DOM), '#language', Array)
#12 /usr/local/apache/common-local/php-1.22wmf8/includes/parser/Preprocessor_DOM.php(1085): Parser->braceSubstitution(Array, Object(PPTemplateFrame_DOM))
#13 /usr/local/apache/common-local/php-1.22wmf8/includes/parser/Parser.php(3432): PPFrame_DOM->expand(Object(PPNode_DOM))
#14 /usr/local/apache/common-local/php-1.22wmf8/includes/parser/Preprocessor_DOM.php(1085): Parser->braceSubstitution(Array, Object(PPTemplateFrame_DOM))
Version: 1.22.0
Severity: normal