  Traditional multitask learning methods basically can only exploit common
knowledge in task- or language-wise, which lose either cross-language or
cross-task knowledge. This paper proposes a general multilingual multitask
model, named SkillNet-X, which enables a single model to tackle many different
tasks from different languages. To this end, we define several
language-specific skills and task-specific skills, each of which corresponds to
a skill module. SkillNet-X sparsely activates parts of the skill modules which
are relevant either to the target task or the target language. Acting as
knowledge transit hubs, skill modules are capable of absorbing task-related
knowledge and language-related knowledge consecutively. Based on Transformer,
we modify the multi-head attention layer and the feed forward network layer to
accommodate skill modules. We evaluate SkillNet-X on eleven natural language
understanding datasets in four languages. Results show that SkillNet-X performs
better than task-specific baselines and two multitask learning baselines (i.e.,
dense joint model and Mixture-of-Experts model). Furthermore, skill
pre-training further improves the performance of SkillNet-X on almost all
datasets. To investigate the generalization of our model, we conduct
experiments on two new tasks and find that SkillNet-X significantly outperforms
baselines.
