Expand, increase, grow, broaden, multiply, fire up, light a fire under, raise, advance, boost, fortify

Certainly! There are several alternative ways to express the idea of "improving your language skills" without always using the word "improve." Here are some options: Enhance your language skills. Refine your language proficiency. Develop your linguistic abilities. Polish your language competency. Elevate your language expertise. Hone your communication skills. Upgrade your language proficiency. Augment your linguistic aptitude. Expand your language capabilities. Perfect your language skills. While "extend" can be used in some contexts related to language skills, it may not be the best synonym for "improve" in this context. "Extend" typically means to make something longer or broader, whereas the phrases I provided above focus more on enhancing or refining your existing language abilities.

‘Improve your language skills’ is grammatically fine, as are the alternatives suggested by others. But they’re vague and nearly meaningless. It’s better to think about what you really are improving and then say that!