My field is language-related and I have been conducting research in it with two professors in the past couple of years. Is it better for me to request recommendation letters from these two professors for my PhD application or should I rely on a more diverse approach? For instance, combining letters from one of these professors with those coming from professors who taught me during the MA years (which, by the way, I graduated from several years ago) or maybe even my MA thesis advisor. I should also mention that it is not very likely for me to apply to universities in the US.
Generally, doctoral programs are designed to produce researchers and academics in a given field. Given this, I suggest that you request letters of recommendation from the faculty members who can best speak to your ability to produce high-quality research. It sounds like you've been working with two professors who might be able to do that.
There are some caveats, though. You want the best possible letters of recommendation you can get. If these two professors with whom you have conducted research are willing to provide excellent LORs, then I'd suggest that you ask them for LORs. But if your relationship with them is strained or if you under-performed while working with them, then perhaps you'll have to look elsewhere.
I'm uncertain why a more "diverse approach" would be advantageous to you. Your LORs should speak to your ability to excel in a PhD program, your capacities for research and original thinking, the likelihood that you'll contribute to the department, and other qualifications.