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I am seriously wondering how to go about writing dialogues for characters whose native language isn't English and who aren't very fluent in English. It's very hard, because people have different levels of fluency, and it may also be kind of offensive to write dialogues with several grammatical mistakes.

Similar questions have been asked in the past, for example How do I make an ESL character sound realistic? and How to write dialogue for someone who is intelligent but barely speaks the language? You might take a look at those.

Let me give you a different approach, however.

Unless the way the characters speak is an actual plot point, it is not unreasonable for people who know they are going to immigrate to a country to put some effort into learning the language in advance. If your characters have done so, their grammar might not in fact be broken. (Realistically, some particular mistakes would still be made, particularly where the language has some exception to the rules. But as a writer, you are free to ignore those.) Realistic language for such a scenario would include short simple sentences, simple words, no colloquialisms. They would have an accent, but this is one element that's better told than shown - it is rather tiresome, and sometimes hard, to read phonetically written accent for more than a line or two.

If your characters have taken language lessons in advance, their real struggle would be with understanding what is being said to them: their teacher would have been talking slowly, and would have either had a local accent, or spoken something close to R.P., whereas upon arrival, they'd be hearing people talking fast, enunciating poorly, and having all kinds of weird accents (Newcastle comes to mind). But that too is something you can gloss over in your writing, if you wish. It might be that because they have an accent and look like foreigners, people make an effort to talk slowly to them. (In fact, in For Whom the Bell Tolls, there's a scene with a character talking to the MC slowly, using very simple phrases, when the MC in fact is a professor of Spanish Language. It's just his appearance that marks him as a stranger.)

Dialogue with severe grammatical mistakes is not offensive (usually), but it gets tiresome very quickly. It doesn't flow, the reader has to struggle through it. If there is any way for you to avoid using more than a few lines of it, try to do so.

If they are fairly well educated people of the middle class, they will probably have a good grasp of the basics of English.

Idioms are always an issue - as with the Casablanca scene. Idioms have an internal logic that is difficult to explain and often puzzling to those who are learning English.

I went to school with an Italian exchange student whose written English was much better than ours. He had learned Standard Written English and had been strongly encouraged to use that as his norm.

Idioms gave him some trouble and his spoken English was formal.

I have also known various refugees from different nations and they often did not know where they were going to end up so had better Spanish, French or German than English.

Your treatment of their dialogue will depend in part on the reason they came to the new country.

Scenario one
Immigrants coming to a new land, probably to join other members of their extended family who already made the move. Such characters might not be too quick to learn English as they plan on living among people of their own nationality. They will eventually learn it, but mixing it with their native tongue when the foreign word escapes them.

Scenario two
Foreign Student/Work permit
These people will have studied English and will have a good grasp of the formal, but can struggle with the colloquial. Conversely, some might have learned English from movies and have an odd vocabulary based on Elvis movies or such.

Scenario three
Refugees - the most varied of the lot as they can be of any socioeconomic group and might be highly educated - doctor or engineer back home but can’t get work as anything but a janitor or cabbie. They might not have known that they were going to an English speaking country - just where the ticket they could afford or were given took them. Their learning of the language will be more incremental and they will be frustrated when their level of English comes nowhere near the level of their thoughts and they must either use their native tongue or be silent on what matters most.

Is there any chance you can work/volunteer at an ESL tutoring center or writing center, especially at a community college? That's a great way to see which errors people of different languages make.

I had a lot of Korean students, and their language lacks articles ("the" "a/an"), so they wouldn't see that they skipped them in English.

Also English has a lot of redundancies, and they hated including them as much as I hate using a double-negative as an intensifier (which Russian does, as well as many dialects of English).