The first Windows Vista Ultimate Service Pack 1 (SP1) will only be available to users who have installed certain language packs, Microsoft has admitted.

The company has said delays in updating more than 30 bonus language packs mean that those using Vista Ultimate with 31 language packs must wait for SP1's "second wave" before installing the update.

Only Vista Ultimate users running the English, French, German, Japanese and Spanish language packs should be able to get SP1 in the first release.

"We will be releasing Windows Vista SP1 in two waves," Microsoft program manager Nick White said in a post to the Vista team's blog.

His warning, in fact, wasn't completely unforeseen. A month ago, when Microsoft said it had wrapped up RTM (release to manufacturing) and was shipping SP1 to duplication, it also spelled out a rough calendar for release.

According to that timetable, Ultimate SP1 would appear as an optional download on Windows Update this month with just the five languages (and as an automatic update mid-April). The full 36 packs should appear "shortly after."

Nor is this the first install glitch caused by the language packs. Last October, Barry Goffe, the director of Vista Ultimate, noted that users had had trouble installing the add-ons, and said an install component had been patched to fix the problem.

In another move, Microsoft is promoting Windows Vista Ultimate via a new site, which also includes special offers for US customers.