It is possible to use many different things as a trademark. While not an exhaustive list these are some of the most common: slogans, letters, numbers, foreign language, and designs. Slogans can be registered if they can standalone when not on a label, are not explanatory/informational, are not a familiar expression, and are not just part of a phrase. Numbers and letters can function as very good trademarks when their combinations have no recognizable meaning. If the combination is a recognizable abbreviation and descriptive of the goods, then a secondary meaning will have to be shown. Foreign language trademarks can be registered as long as their translated meaning is not a generic term. It is important to note that when determining confusion with other marks, foreign language marks will only be translated if the average person would know its meaning. Designs can be registered so long as they are not ornamental, background material, or ordinary shapes. A strong design trademark can leave a commercial impression by itself.
This blog is not legal advice and is not specific to your application. You should always consult an attorney.