How can I tell her that I'm willing to listen to her if she wants to talk about her mother, but leave her the option to say no?
Pick a time when you are about to leave (preferably one where you could stay as leaving is optional; e.g. a run to the grocery store rather than leaving for work) and tell her then. In English rather than español, I might say
Hey, I'm heading out to [whatever]. I don't want to force you to do anything you don't want to do, but I want you to know that if you want to talk about your mother, I'm here to listen.
Then kiss her on the forehead (or some other act of physical affection that is culturally and personally appropriate) and leave.
If she seems eager to talk, you can stay and listen instead. But this way, the pressure is off. She can just let you leave if she doesn't want to talk at the moment.
When you get back, you don't need to say anything, but make sure that you express affection physically again (kiss, hug, whatever feels right) and she knows you have returned. She may want to talk then. She's had some time to think about it. If there's something that she wants to say, you're there. If not, you're still there.
Keep your eye out for inexpensive things that you can buy as random gifts. The ideal is something that is fiscally cheap but well tailored to her interests. For example, if she is a big fan of unicorns, buy her a unicorn figurine or poster. "I saw this and thought of you." The idea is to express that you value her. You spend your time when you're not with her thinking about her.
If you have any dead relatives, consider visiting their graves. Ask her if she wants to go.
I've been thinking about [whatever relative] recently and would like to visit the grave at [wherever]. Would you like to go with me?
If she goes with you, you can put flowers on the grave or whatever is culturally appropriate. She may suggest stopping at her mother's grave as well, and you can go with her. This can work because it is you burdening her, so she may feel more comfortable burdening you back.
After this, set aside appropriate times when you both are free. Ask her what she wants to do with the time. Some of the time, she may want to go visit her mother's grave. The goal here is to set aside the time as being together, so she won't feel like she is burdening you with her mourning. And if she wants to watch television or play video games, that's fine too. Leave it open ended so she chooses. If she doesn't choose anything, just hang out until she starts doing something. Then help her with that.
You also may want to think up some anecdotes about that relative. So if she opens up, starts talking, and then stops, you have something to keep the conversation alive. Try to match her emotions. If she's telling happy anecdotes, share one of your own. If she's feeling guilty, try for an anecdote of you doing the same kind of thing. The approach is "I think everyone faces problems like that." If she's feeling scared without her mother to fall back to, share some of your life worries (not about her).
If she's worried about being a burden, look for tasks where you could use her help. Gender-traditional tasks include cooking and cleaning. E.g. bake cookies to take into work to share. That may or may not be appropriate here. I don't know her, so I can't say precisely. But look for things that she is better at doing than you are and try to create situations where it is appropriate for you to ask her help. You should still do most of the work though, not just give her extra tasks.
Consider volunteering somewhere to do something that she will regard as a social good. Ask her to go with you. Church groups can often help with things like that, but there are non-church groups as well (e.g. environmentalists or other political causes). Again, this should be something that she will find fulfilling. I don't know what that is, but hopefully you do. If not, consider asking a friend or relative of hers. The goal here is to make her feel valuable to the world. To think about problems beyond her own.
and the few words I said didn´t really help her that much- what did you say?