Error when paring environment variable inside embed tag about stash HOT 1 CLOSED

That means the {user_language} variable hasn't been replaced with its value at the point the {exp:stash:set...} tag runs. You can use parse="inward" to tell a {exp:...} tag to parse variables passed in parameters.

{exp:stash:set_value name="top-menu-{user_language}" value="{stash:embed:partials:main-menu-{user_language}}" parse="inward"}

If I remember correctly (it's been a while!), you could also do without the set_variable and use an explicit name and filename to load your embed file into a variable:

{exp:stash:embed name="top-menu-{user_language}" file_name="partials:main-menu-{user_language}}" parse="inward" output="no" process="inline"}

from stash.