In general, my preference would be for the following system. In descending order of preference, we should generally:
- Use English name. If there is an official English name which fits in 35 characters, we should go with that. Subtitles can be omitted in general.
- Use English abbreviation. If the English title exists, but it does not fit in 35 characters, and there is an official abbreviation of the English title which does fit, then use that.
- Use romanized Japanese title. Hepburn romanization is the standard method most of us use. Since macrons are not available without going to unicode characters (which is a bad idea) be sure to do these in the traditional style, e.g. ō should be replaced with ou. In Japanese word boundaries are sometimes ambiguous, but usually it's fairly clear where they should be.
- Use Japanese abbreviated title. This is going to be common for shows with long titles, like Ore no Imōto ga Konna ni Kawaii Wake ga Nai gets tagged as oreimo. There are usually official abbreviations for series with very long titles, at least for those series that are fairly successful.
- If none of the above seem to fit, come up with something that works temporarily, e.g. the first N words of the title. Make a meta post asking for suggestions for a better tag name.
We can't expect all cases to be handled by the above rules, and even when they are it may not give the best name. If the English name is official but very uncommon we might want to stick with the Japanese one. In these cases, it's best to make a meta post asking for suggestions. These will have to be treated on an individual basis.
Also, tag synonyms are very helpful. Having lots of synonyms hurts nothing so long as we aren't going to be needing that tag name for anything else (unlikely). Unfortunately tag synonyms are often hard to create for smaller tags, which is where they're more useful, so it's probably easier just to PM a mod in chat for obvious requests, or make a meta post requesting a synonym if there could be some ambiguity or there's a need to document and vote on the request. Eventually the tag synonym suggestions should work the same way they do on the larger sites, but right now we don't have the volume of posts in most tags for it to be feasible.
Finally, in all cases except the totally obvious ones, the tag wiki (not necessarily excerpt) should contain all of the titles by which a series is known, near the top. That will help to clear up some confusion. An image can also go a long way to confirming that the tag you're using is/isn't correct.