If you decide to put foreign words in your fiction, it's best to italicize it the first time, but after that, leave it alone/don't italicize it.
When defining the foreign words in our prose, it's best not to stop everything for the definition.
Here's an example of what not to do:
He swung the parma. A short curved sword.
It's obvious that the writer it "telling" us what a "parma" is. Blaaack! All this does is remind our readers that they're "reading," pulling them out of the story by creating a speed bump.
Here's how to do it instead. The following keeps the story moving forward and doesn't stop the action.
He swung the parma, the short curved sword sliced into the enemy's armor.
See how we've defined the word "parma" without stopping the pace, without holding up the action? I love it when writers do this!
In the following example we have a Native American boy who is half white, speaking/thinking in Cheyenne.
He tied the knot fast and rubbed his hand along the soft fur. The skins would make a good muff for Grandmother this winter. He’d seen many white women wear them; they looked warm and his heveškemo deserved the best.
Notice here how we define the word "Grandmother" in the last line of the text.
He could still feel Mamma's hand in his. Could feel her letting go as the soldiers pulled her away. Could feel her stola ripping as he clutched it. All he had left was the shredded fabric from her dress still in his hand.
Here we know that stola means dress.
Titus ordered the new slaves to stand in the center of the atrium next to the impluvium. David ran his hand along the smooth edge of the large marble fountain that collected rainwater from an opening in the roof.
After the slaves were led away, and she was safe from being seen by Grandfather, she crept into the tablinum. She slipped through the heavy crimson curtain and into the small room between the atrium and the indoor courtyard.
She held her breath when she saw Grandmother and Aunt Fabia kneeling before the shrine. Usually they prayed in the morning.
Notice here how tablinum is defined in two paragraphs. The first by saying it's a small room between the atrium and the indoor courtyard. The second by showing what it's being used for: they're kneeling before the shrine. So, the reader sees that it's not only a small room, but it's a room with a shrine and where people pray.
Last but not least:
Before Grandmother could respond, Alethea slipped through the curtain on the other side of the tablinum and entered the peristyle. Flowers filled the indoor courtyard with a sweet fragrance. She especially liked the jasmine and breathed in its scent as she wandered between the plants and marble statues, following the patterns of flowers in the colorful mosaic on the floor.
This not only defines the peristyle, it shows us what it is.
Notice in all these examples how the writer also captures the emotion of the character/scene, rather than just stating a word's definition.
So, if you have foreign words in your manuscripts, have fun with them, don't just define them. Get creative, make them "move."
Tip: This can also apply to difficult words in children's fiction.