I am a native speaker of Japanese. Although I have a casual interest in user interface, I have no expertise in user interface.
I do not know the cause of your results of A/B testing. For what it’s worth, I will list the small oddities of the Japanese pages which I noticed. I will not imply that anything listed here is the cause of the result which you observed. You should figure out the real cause by yourself if necessary.
Quality of translation. The quality of the Japanese translation is ok but not excellent:
- On the first page, the translation “それは無料です。インストールに必要なのはたったの1分です。” for “It’s free. Takes just 1 minute to install.” is correct but unnatural (especially the first sentence). It sounds translation-y. I would translate the first sentence to just “無料です。” instead of “それは無料です。”.
- On the second page, the explanation of Step 1 is translated incorrectly. The Japanese text says “Check ‘save file’ and click ‘ok’ to download the Veetle TV installer,” which is different from what the English text says. It is also inconsistent with the screenshot.
- Added: On the first page, the word “channel” is translated into “チャネル.” This form is not rare, but I think that the form “チャンネル” is more common when we refer to a collection of video or audio programs (for example, YouTube uses “チャンネル”). The use of the less common form might add slight awkwardness to the page.
Partial translation. I agree to Oskar Duveborn that only small portion of a page being translated to my language reminds me of ads.
Added: Choice of font. When I opened the page with Internet Explorer, the Japanese text did not look right, because the browser showed part of the Japanese text in the SimSun font, which is a font for simplified Chinese. However, I am using the English version of Windows, and I do not know if it happened because of this. If the same issue happens also on the Japanese version of Windows, it is very likely that it gives a negative impression of your website to Japanese speakers.
Added: Alignment of captions of buttons. As you state in a comment on the question, the Japanese instruction lacks the arrow icons on the buttons. I do not think that the absence of the icons per se has any negative impact, but as a result, the texts on the buttons are aligned to the left instead of centered for no apparent reason. This is a little awkward.