。
She's always complaining about her low pay.
Literal
She [topic-は] low-salary [about-に] always complaint [object-を] is-saying.
不平を言う ('to voice complaints, gripe') is the standard idiom for grumbling. 不平 ('discontent, grievance') is more formal than 文句 ('complaint, gripe'), but the colloquial 文句を言う carries the same shape. に on 安月給 marks the target of the complaining. 安月給 ('low monthly salary') is built from 安い ('cheap, low') + 月給 ('monthly salary') — Japanese pay is often discussed in monthly rather than hourly or annual terms, reflecting the salaried-employee norm. ~ている here gives the habitual reading: she does this all the time.