How do I ensure that user fills in a field in certain language?

You can do that by using the validation feature by adding a regular expression, checking that the input is in a certain language code range.

For example if you want to make sure the language is in Cyrillic, you can use:

[\u0400-\u04FF\u0500-\u052F]

Below is a list of language code ranges for each languages. You can also go to this link for more references.

0000-007F Basic Latin

0080-00FF Latin-1 Supplement

0100-017F Latin Extended-A

0180-024F Latin Extended-B

0250-02AF IPA Extensions

02B0-02FF Spacing Modifier Letters

0300-036F Combining Diacritical Marks

0370-03FF Greek and Coptic

0400-04FF Cyrillic

0500-052F Cyrillic Supplement

0530-058F Armenian

0590-05FF Hebrew

0600-06FF Arabic

0700-074F Syriac

0750-077F Arabic Supplement

0780-07BF Thaana

07C0-07FF NKo

0900-097F Devanagari

0980-09FF Bengali

0A00-0A7F Gurmukhi

0A80-0AFF Gujarati

0B00-0B7F Oriya

0B80-0BFF Tamil

0C00-0C7F Telugu

0C80-0CFF Kannada

0D00-0D7F Malayalam

0D80-0DFF Sinhala

0E00-0E7F Thai

0E80-0EFF Lao

0F00-0FFF Tibetan

1000-109F Myanmar

10A0-10FF Georgian

1100-11FF Hangul Jamo

1200-137F Ethiopic

1380-139F Ethiopic Supplement

13A0-13FF Cherokee

1400-167F Unified Canadian Aboriginal Syllabics

1680-169F Ogham

16A0-16FF Runic

1700-171F Tagalog

1720-173F Hanunoo

1740-175F Buhid

1760-177F Tagbanwa

1780-17FF Khmer

1800-18AF Mongolian

1900-194F Limbu

1950-197F Tai Le

1980-19DF New Tai Lue

19E0-19FF Khmer Symbols

1A00-1A1F Buginese

1B00-1B7F Balinese

1D00-1D7F Phonetic Extensions

1D80-1DBF Phonetic Extensions Supplement

1DC0-1DFF Combining Diacritical Marks Supplement

1E00-1EFF Latin Extended Additional

1F00-1FFF Greek Extended

2000-206F General Punctuation

2070-209F Superscripts and Subscripts

20A0-20CF Currency Symbols

20D0-20FF Combining Diacritical Marks for Symbols

2100-214F Letterlike Symbols

2150-218F Number Forms

2190-21FF Arrows

2200-22FF Mathematical Operators

2300-23FF Miscellaneous Technical

2400-243F Control Pictures

2440-245F Optical Character Recognition

2460-24FF Enclosed Alphanumerics

2500-257F Box Drawing

2580-259F Block Elements

25A0-25FF Geometric Shapes

2600-26FF Miscellaneous Symbols

2700-27BF Dingbats

27C0-27EF Miscellaneous Mathematical Symbols-A

27F0-27FF Supplemental Arrows-A

2800-28FF Braille Patterns

2900-297F Supplemental Arrows-B

2980-29FF Miscellaneous Mathematical Symbols-B

2A00-2AFF Supplemental Mathematical Operators

2B00-2BFF Miscellaneous Symbols and Arrows

2C00-2C5F Glagolitic

2C60-2C7F Latin Extended-C

2C80-2CFF Coptic

2D00-2D2F Georgian Supplement

2D30-2D7F Tifinagh

2D80-2DDF Ethiopic Extended

2E00-2E7F Supplemental Punctuation

2E80-2EFF CJK Radicals Supplement

2F00-2FDF Kangxi Radicals

2FF0-2FFF Ideographic Description Characters

3000-303F CJK Symbols and Punctuation

3040-309F Hiragana

30A0-30FF Katakana

3100-312F Bopomofo

3130-318F Hangul Compatibility Jamo

3190-319F Kanbun

31A0-31BF Bopomofo Extended

31C0-31EF CJK Strokes

31F0-31FF Katakana Phonetic Extensions

3200-32FF Enclosed CJK Letters and Months

3300-33FF CJK Compatibility

3400-4DBF CJK Unified Ideographs Extension A

4DC0-4DFF Yijing Hexagram Symbols

4E00-9FFF CJK Unified Ideographs

A000-A48F Yi Syllables

A490-A4CF Yi Radicals

A700-A71F Modifier Tone Letters

A720-A7FF Latin Extended-D

A800-A82F Syloti Nagri

A840-A87F Phags-pa

AC00-D7AF Hangul Syllables

D800-DB7F High Surrogates

DB80-DBFF High Private Use Surrogates

DC00-DFFF Low Surrogates

E000-F8FF Private Use Area

F900-FAFF CJK Compatibility Ideographs

FB00-FB4F Alphabetic Presentation Forms

FB50-FDFF Arabic Presentation Forms-A

FE00-FE0F Variation Selectors

FE10-FE1F Vertical Forms

FE20-FE2F Combining Half Marks

FE30-FE4F CJK Compatibility Forms

FE50-FE6F Small Form Variants

FE70-FEFF Arabic Presentation Forms-B

FF00-FFEF Halfwidth and Fullwidth Forms

FFF0-FFFF Specials