Use Plural-Forms strings
Strings that can be presented in both singular and plural forms are currently being hardcoded with "s" and "ies", which is bad for localization. This is true for:
libwget/cookie.c: debug_printf(_("Saved %d cookie%s into '%s'\n"), size, size != 1 ? "s" : "", fname);
libwget/hpkp.c: debug_printf("Saved %d HPKP entr%s into '%s'\n", size, size != 1 ? "ies" : "y", hpkp_db_priv->fname);
libwget/tls_session.c: debug_printf(_("Saved %d TLS session entr%s into '%s'\n"), size, size != 1 ? "ies" : "y", fname);
libwget/hsts.c: debug_printf(_("Saved %d HSTS entr%s into '%s'\n"), size, size != 1 ? "ies" : "y", hsts_db_priv->fname);
Verbs in other languages can have different forms in singular and plural forms, and the current method to fill in 's' or 'y'/'ies' can make it possible to achieve a proper localization.
Please implement Plural-Forms strings. More information on this can be read in https://www.gnu.org/software/gettext/manual/html_node/Plural-forms.html