Nesting Variables do not work for me on a raspberry

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I tried to do things like this :

LA="de"
COUNTRY="DE"
LANG="${LA}_${COUNTRY}"
CHARSET="UTF-8"
LC_CTYPE="${LANG}@${CHARSET}"

and it does not work on my raspberry the variables are simply not loaded.

But things like this :

LA="de"
COUNTRY="DE"
LANG="de_DE"
CHARSET="UTF-8"
LC_CTYPE="de_DE@UTF-8"

work without problems on raspberry

on a normal Linux box both versions work.

Is there some requirement missing on the raspberry? how can I put phpdotenv in debug mode or see the phpdotenvs log files ?