What suggestions should I make when reviewing a poorly written paper?
To comment on language is fair, to suggest improvements and even making corrections goes beyond what can be expected, depending on the degree of problems. There is of course a fine line between when something can be salvaged with a little editing and where things start to lose meaning. Many journals and publishers provide services (albeit often at a cost) for non-native speakers. What you can do is to try to help the author(s) if possible by providing examples and making minor corrections. But, it is not your job to be a service. You should state your opinion about the paper, clearly separating the scientific aspect (indicate if your think the science holds) and the language issue. It is particularly potent to state when the language obscures the scientific message. In the end it is the editor who should decide what must be done.