To select a global language from the command line, say ‘-Ll’,
where l is one of {c,c++,n,n9,r,r9,v,x}. See Languages.
Usually, the global language is set via an ‘@’ command in limbo, not on the command line. However, one may need to use a command-line option such as ‘-L_’ if a subsequent command-line option is language-dependent. See, for example, the discussion of the option ‘-D’ in -D_.