If I need to make up a language, do I need to make up their grammar rules too?
I guess it would depend on the language. If you will constantly be translating it into English, you can type pretty much whatever you want, and just make sure that the English translation is grammatically correct.
I would advise that you at least makeup basic grammar rules, but you don't have to do anything big like case systems or Honorifics. How much Grammer you do should depend on how much you want it to be different from English and how much you plan on using the language. I hope I was of some help and your language endveves go well.
It really depends on what you will be using the language for. If it’s just to name characters and places, you only need a few grammatical aspects, like how they add description to nouns (do they say Blue Mountain, or Mountain Blue, or Mountain of Blue, etc) and whether names get prefixes/suffixes for things (for example, a lot of English names have a feminine form: Henry/Henrietta, Daniel/Danielle, Duke/Duchess; Spanish words have a diminutive: Señora/Señorita; Japanese have the -chan and -san honorifics). If you do this, your names are more likely to have something in common and thus sound like they’re from the same language.
Think about surnames. Van Gogh and Van Helsing look like they come from the same language. Or LaCroix, Leroux, DuPont, Descartes <— all French. If you have one character called Natalinette, another called Kong-Li, and they live in the town of Zkanthiidc in the country of Scourge, the viewer will assume that none of those names comes from the same language.
If you want to write sentences in this language, then yes, you need some kind of basic grammar. Of course, you can just write gibberish and then ‘translate’ into English, but if you want words to have definite meanings in your sentences, basic grammar is best.