Restricting search on one language
I do the same and it works well. The only problem is in searching all messages at translatewiki.net: we don't have a group for that.
To search all groups it's best to use search, but it's only feasible for uncommon words, otherwise you hit Evil's problem.
One nagging feature of search is that if we set search results to 500 (and use the browser to search for the language code), then change 10 messages so they no longer match the search, the next page of 500 results is shifted by 10. In other words, the 10 that should be at the top of the next page are not there; they've been moved to the previous page, that we just moved away from. So we'll miss them.
Another nagging feature is that it doesn't do partial matches. To replace a word completely, you need to search all its variations: singular, plural, masculine, feminine, present tense, past tense, future tense and combinations of those. It drives me NUTS! :) And I always miss some, of course.