。
She's been getting better at English.
Literal
She [topic-は] English [genitive-の] strength [object-を] put-coming is.
力をつける is a productive set phrase: 'put on power / gain ability,' applied to skills, knowledge, athletic strength — anywhere capability accumulates. The compound auxiliary ~てくる adds the sense of 'a change unfolding up to now,' so 力をつけてくる reads as 'has been gaining capability.' Wrapping that in ~ている then signals the change is ongoing rather than complete: 'has been getting stronger and is still doing so.' Stacking ~てくる + ~ている this way is the standard shape for describing a continuing trend: 太ってきている ('is getting fatter'), 慣れてきている ('is getting used to it').