I haven't attempted to do anything with Gettext yet, and won't be in a position to tackle this for a while. Having had some discussions with the Bedwyr Language Institute at Bangor University about the plural rules (they requested the changes at CLDR), it appears that most commercial websites and computer programs have in the past just not put any plural rules for Welsh onto their systems. Since the mutation rules are complex, it appears that some linguists have opted to not mutate at all, just using the singular form of a word in an unmutated state, analogous to websites in English which use 'him' to refer to 'him/her'. Given that that is the case, it might prove impossible to get Gettext to follow the CLDR and translatewiki.net pattern. How big a problem is it likely to be in future if Gettext continues to be out of step with CLDR for Welsh?
OK. If that is so, am I right in thinking that for the projects here which use Gettext, we can use the 6 plural rules for Welsh, same as currently used on MediaWiki and CLDR?