My computer's default language is Portuguese, and I don't want to change that. But, for some usages (especially programming) it's better to have English output, for example to copy/paste an error message on a search engine.
I would like to have emacs in English only, to have for example eshell or built-in gdb output in English.
It sounds easy but I didn't find out how to do. Is there a simple way to do that?
I would like to have emacs in English only,
Emacs is already English only.
to have for example eshell or built-in gdb output in English.
gdb output is not builtin to Emacs, it comes from gdb. By eshell, I assume you refer to external programs which you call from eshell.
On a Unixish system, you can tell programs to use non-localized message and settings by setting the environment variable LC_ALL. To set that in Emacs, use
(setenv "LC_ALL" "C")
C is the standard non-localized locale. See also
This thread seems relevant. Emacs does not (yet) provide a way to configure it for a given natural language.
You can, however, configure the Emacs calendar to use local holidays and such. See Calendar Localization on Emacs Wiki.
But please do point to specific things that appear in Portuguese with your Emacs release, in case someone here has a suggestion for how to make that part use English instead.
