A computer-implemented method for assisting a user to input Vietnamese text to a user device lacking a subset of characters in a Vietnamese alphabet includes receiving a character input by a user, determining three words previously input by the user, the three words having already had diacritical marks inserted, transmitting the three words and the character to a server via a network, receiving first and second information corresponding to the character from the server via the network, the first and second information generated at the server based on a context of the three words, the context determined at the server using a language model, the first and second information indicating whether the character requires a diacritical mark and a specific diacritical mark, respectively, generating a modified character comprising a character in the Vietnamese alphabet based on the character and the first and second information, and displaying the modified character.