Comment 4 for bug 549101

Neither of them is of fault. The LANGUAGE environment variable setting should have precedence over LANG. But some applications don't honor the LANGUAGE variable and only take LANG into account. Therefor those applications are buggy.

Therefor, if you set your environment variable to English (UK) and your Desktop language to Spanish with fallback to English, all applications are expected to use Spanish and only fall back to English when a Spanish translation is not available.

gdm only sets the LANG variable (environment), while language-selector lets you set LANG and LANGUAGE. That's a different issue and a bug is open for that.