I won't look into the DTML Parser code right now. Don't need no headache ;-) But this is my first guess:

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<dtml-if getEasyLanguageService>

DTML looks at "getEasyLanguageService" and does an implicit "getEasyLanguageService()". So now "getEasyLanguageService" (as an attribute in the DTML namespace) represents the actual service object the method should return.

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<dtml-let ELS="getEasyLanguageService">

ELS represents the service object now

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<dtml-in getAllLanguages> <dtml-var "ELS.getLanguageByRFC3066(_['sequence-item'])"

the getLanguageByRFC3066() method of the service object is called And here it fails because:

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<dtml-if "is_language_aware() and getEasyLanguageService()">

getEasyLanguageService() is called explicitly, and in the "", so DTML does no magic. It does NOT put the returned service object into the namespace as "getEasyLanguageService" (as it would do in the first example)

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<dtml-let ELS="getEasyLanguageService">

the method "getEasyLanguageService", not its return value (i.e. the language service object) is passed to ELS

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<dtml-in getAllLanguages> <dtml-var "ELS.getLanguageByRFC3066(_['sequence-item'])"

This fails because Zope tries to get the getLanguageByRFC3066() method of the method (or function) getEasyLanguageService (without ()), and that is not allowed here. Yep Joachim