There are cruise lines marketed primarily in Europe and in Asia, aboard which English is not the primary language. It usually takes some extra effort to locate these lines because they are not marketed very extensively in the United States, though they usually do accept bookings from American travel agents. Here are some examples.
>> Costa Crociere, a division of Carnival Corporation, markets several ships primarily in Italy, France, and Spain. On those cruises, the majority of passengers would speak Italian, French, and Spanish.
>> Aida Cruises/Seetours and A'Rosa, both divisions of P&O Princess Cruises Plc., market their cruises primarily in Germany. The language onboard is, of course, German.
>> Festival Cruises, AKA First European Cruises here in the States, markets cruises primarily in Greece. The primary language probalby would be Greek.
>> Star Cruises operates primarily in several Asian markets, so the majority of passengers would speak the respective Asian languages.
This is by no means an attempt to provide an exhaustive list, but I can't make a specific recommendation because you did not say which language(s) your mother does speak. I would recommend that you explore these options, though, as they would serve your mother's needs more effectively than any cruise line marketed primarily in North America.
The best source of information about such cruises probably would be an international cruising guidebook, such as the one published by Berlitz, which you can buy at any major bookstore.
That said, I should note that some cruise lines, including Princess Cruises, do provide dining menus, daily schedules, and other materials in the languages that are primary to a significant number of their passengers. If you are flexible and willing to work with the line, you probably can identify specific itineraries that would have significant contingents from a particular country or that would speak a particular language, and then book on one of those itineraries.
Of course, another option is to arrange a group cruise for a social, civic, or religious group to which your mother belongs. If you do so, they would form a social core who speak your mother's language during the cruise.