The proper noun Portuguese is an uncountable (mass) noun, as a word for a person of Portugal or the people of Portugal; and a word for the language of Portugal.
The possessive form of the noun Portuguese is Portuguese's.
The plural form of the word "Portuguese" is "Portuguese." It remains the same in both singular and plural forms.
Coaster's is singular possessive. The plural is coasters, the plural possessive is coasters'.
The correct form is "its" for the possessive form in the plural. "Its" is used for both the singular and plural possessive forms, without an apostrophe.
The singular possessive of "ant" is "ant's" and the plural possessive is "ants'".
The plural is briefs. The plural possessive is briefs'.
The plural is lads. The plural possessive is lads'.
The plural is trollies. The plural possessive is trollies'.
The plural is stations. The plural possessive is stations'.
The plural is relatives. The plural possessive is relatives'.
Museum is singular. Museum's is singular possessive. Museums is plural. Museums' is plural possessive.
The singular possessive form is heart's; the plural possessive form is hearts'.
The plural form is homes; the plural possessive is homes'.