At my company we've been dreaming of being able to localize our UIs since I started working there. While we have localized here and there for the most part it just doesn't happen. There simply aren't the resources or knowledge.
Instead our employees in all branches are required to speak English. I have no idea how branches determine whether a person's ability is up to standard to be hired. I've never seen any guidelines for branch management to follow and admittedly our industry is one where you have to learn a lot of jargon, too.
I imagine that may other global enterprises are the same way. This means that your users may struggle to understand your UI, especially those newly hired employees. So part of your user research should be pointed at determining at what level your users can understand written English.
Here are guidelines you can use based on the European Framework of Reference for Languages. (See Chapter 9 for more information.) I assigned the reading levels based on this chart at Benchmark Education. This will allow you to use tools like WebpageFX's Readability Test Tool when writing your labels and messaging.
|Language Level||Reading Level||Ability Description|
|Beginner - A1||K-1||
|Beginner - A2||2-3||
|Intermediate - B1||3-4||
|Intermediate - B2||5-6||
|Advanced - C1||6-7||
|Advanced - C2||8-9+||