If your port requires gettext, just set USE_GETTEXT to yes, and your port will grow the dependency on devel/gettext. The value of USE_GETTEXT can also specify the required version of the libintl library, the basic part of gettext, but using this feature is strongly discouraged: Your port should work with just the current version of devel/gettext.
A rather common case is a port using gettext and configure. Generally, GNU configure should be able to locate gettext automatically. If it ever fails to, hints at the location of gettext can be passed in CPPFLAGS and LDFLAGS as follows:
USE_GETTEXT= yes
CPPFLAGS+= -I${LOCALBASE}/include
LDFLAGS+= -L${LOCALBASE}/lib
GNU_CONFIGURE= yes
Of course, the code can be more compact if there are no more flags to pass to configure:
USE_GETTEXT= yes GNU_CONFIGURE= yes
Some software products allow for disabling NLS, e.g., through passing --disable-nls to configure. In that
case, your port should use gettext conditionally,
depending on the status of WITHOUT_NLS. For ports of low to medium complexity, you
can rely on the following idiom:
GNU_CONFIGURE= yes
.include <bsd.port.options.mk>
.if ${PORT_OPTIONS:MNLS}
USE_GETTEXT= yes
PLIST_SUB+= NLS=""
.else
CONFIGURE_ARGS+= --disable-nls
PLIST_SUB+= NLS="@comment "
.endif
.include <bsd.port.mk>
The next item on your to-do list is to arrange so that the message catalog files are included in the packing list conditionally. The Makefile part of this task is already provided by the idiom. It is explained in the section on advanced pkg-plist practices. In a nutshell, each occurrence of %%NLS%% in pkg-plist will be replaced by ``@comment '' if NLS is disabled, or by a null string if NLS is enabled. Consequently, the lines prefixed by %%NLS%% will become mere comments in the final packing list if NLS is off; otherwise the prefix will be just left out. All you need to do now is insert %%NLS%% before each path to a message catalog file in pkg-plist. For example:
%%NLS%%share/locale/fr/LC_MESSAGES/foobar.mo %%NLS%%share/locale/no/LC_MESSAGES/foobar.mo
In high complexity cases, you may need to use more advanced techniques than the recipe given here, such as dynamic packing list generation.
There is a point to note about installing message catalog files. The target directories for them, which reside under LOCALBASE/share/locale, should rarely be created and removed by a port. The most popular languages have their respective directories listed in PORTSDIR/Templates/BSD.local.dist. The directories for many other languages are governed by the devel/gettext port. Consult its pkg-plist and see whether the port is going to install a message catalog file for a unique language.