今日のランチはラーメンと半チャーハンでしたって英語でなんて言うの？

It's usually okay to just say "ramen" just like that for 「ラーメン」, but for 「半チャーハン」there are a few different phrases you could use:
- half-size fried rice
- half-portion of fried rice

To tell about what you ate, you can use "had" or "ate," both are totally okay.
- I ate ramen and a half-size fried rice for lunch today.
- I had ramen and a half-size fried rice for lunch today.
- For lunch I had ramen and a half-size fried rice.

You can say "I ate ramen noodles and a small portion of fried rice for lunch today." A small portion is a way of explaining that you had a half-sized amount.

"I had some ramen and a small fried rice for lunch today." You can always say the word "half-sized" or use small, medium, or large.
"For lunch today I ate some ramen and a little bit of rice. " If your rice isn't a normal size you can say a little bit, or a lot. If it just rice you can say, "For lunch today I am some ramen and rice."

example
"I ate some noodles and rice today for my lunch."
or
"I had fried rice and noodles today."
or
"I had some noodles and fried rice today."

Half a bowl of something, or half a plate of, describes eating half the size of a regular portion of food.

In this case the words 'ate' and 'had' are interchangeable that is to say either may be used without changing the meaning. We don't use 'half-size' when talking about food. We can use 'half-portion' instead. We may also use 'small portion of rice' or 'some rice' or 'a little rice', all are acceptable and will be understood.

Ramen is a mixture of fish soup and perhaps seaweed and pork or other meat along with noodles and possibly eggs - and you had rice with it as well! In fact, to an English mentality, it seems like the whole contents of a kitchen have been emptied into a soup pan! Why are there no potatoes also? Best just to say 'Japanese broth.'