Issue #8524: add or change some missing or duplicate mnemonics.

plug-ins/common/file-png.c

@@ -272,7 +272,7 @@ png_create_procedure (GimpPlugIn  *plug_in,
                              G_PARAM_READWRITE);
 
       GIMP_PROC_ARG_BOOLEAN (procedure, "optimize-palette",
-                             _("Optimize for smallest possible palette size"),
+                             _("_Optimize for smallest possible palette size"),
                              _("When checked, save as 1, 2, 4, or 8-bit depending"
                                " on number of colors used. When unchecked, always"
                                " save as 8-bit"),

plug-ins/file-tiff/file-tiff.c

@@ -208,7 +208,7 @@ tiff_create_procedure (GimpPlugIn  *plug_in,
                                           "tif,tiff");
 
       GIMP_PROC_ARG_BOOLEAN (procedure, "bigtiff",
-                             "Export in BigTIFF variant file format",
+                             "Export in _BigTIFF variant file format",
                              "The BigTIFF variant file format uses 64-bit offsets, "
                              "hence supporting over 4GiB files and bigger",
                              FALSE,
@@ -230,7 +230,7 @@ tiff_create_procedure (GimpPlugIn  *plug_in,
                              G_PARAM_READWRITE);
 
       GIMP_PROC_ARG_BOOLEAN (procedure, "cmyk",
-                             "Export as _CMYK",
+                             "Export as CMY_K",
                              "Create a CMYK TIFF image using the soft-proofing color profile",
                              FALSE,
                              G_PARAM_READWRITE);