Extract gettext strings from code files. More information: https://www.gnu.org/software/gettext/manual/html_node/xgettext-Invocation.html.
messages.po
:xgettext path/to/input_file
xgettext --output path/to/output_file path/to/input_file
xgettext --join-existing --output path/to/output_file path/to/input_file
xgettext --omit-header path/to/input_file