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I wish to keep the language in a command


However, \testlang is not expanded, and i get the error

! Package babel Error: You haven't defined the language testlang yet.

I've seen many tricks related to macro expansions, but I don't understand them well enough to know how to apply them in this scenario.

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up vote 6 down vote accepted

You can expand the argument manually using the following techniques. The easiest thing would be to define your own wrapper environment.

% Expand the argument once:

% Alternative: argument is fully expanded

% usage    
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Thank you. Is there a reason why otherlanguage shouldn't try to expand this by default? – Mikael Öhman Apr 5 '11 at 13:40
@Mikael Öhman: It's much saver to not expand it inside \edef. If it contains things which can't be expanded but must be executed, like assignments, than it will break inside \edef. This is very similar to the fragile vs. robust issue in sectioning commands. – Martin Scharrer Apr 5 '11 at 13:50

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