Perhaps the occasional reminder on the log window like the LFRP reminder can help without the obnoxious flashing colours?
This reminds the speaker, but it would not address the second part of the problem Gavin Mace described:
People often forget they are on (thus continuing to blabber in a different language) or skim over them and respond to things they cannot actually understand.
Personally, I don't think there's any issue with the current tags. My PC will sometimes forget and continue for one statement, but I immediately correct with a // .
As for the second, there's no way of telling why a PC does that. (They may be newbies who don't understand the system, or they're not considering they don't understand the language. Maybe they're playing while intoxicated.) But if you're reading text closely enough to be able to respond meaningfully to it, you can hardly miss the tag as it is.
To add to that, what's being said is at least as important as the language used. Hence, it doesn't make visual sense to me to highlight the language with color to the detriment of the surrounding text. Inattentive players should pay better attention, rather than inflicting attentive players with this distraction from actual content.